Drop /Applications, /Library and /Volumes; make the visibility tests portable

The node table is the Filesystem Hierarchy Standard's paths plus the macOS
directories that carry FHS content under another name. /Applications,
/Library and /Volumes are none of those: they are macOS's own, have no
counterpart in the FHS, and the Finder shows them already, so offering to
manage them was offering something this project has no business managing.
Removing them from the table removes them from the menu, the preference pane
and `fhsctl vis` at once, since all three read that one list.

The visibility tests no longer assume a directory exists. /opt is the
instructive case: it is not part of a stock macOS install at all - Homebrew
creates it - so asserting it exists and is changeable makes `make check` fail
on any machine without Homebrew, for a reason unrelated to this code. CI
passes today only because GitHub's macOS images ship Homebrew preinstalled.
The three changeable nodes are now asserted conditionally, which keeps the
property worth pinning - a node that is present classifies as measured, so an
OS change is still caught - without testing the runner's image.

Author: somestupidgirl

src/visibility/fhs_visibility.c

@@ -29,16 +29,20 @@
  * Alphabetical, as the Finder and the menu present them - not grouped by
  * origin, because a user looking for a directory knows its name and not
  * whether macOS or this layer put it there.
+ *
+ * The list is the Filesystem Hierarchy Standard's, plus the macOS directories
+ * that carry FHS content under another name. /Applications, /Library and
+ * /Volumes are deliberately absent: they are macOS's own, have no counterpart
+ * in the FHS, and the Finder shows them already, so presenting them here would
+ * be offering to manage directories this project has no business managing.
  */
 const struct fhs_node fhs_root_nodes[] = {
-	{ "/Applications", false },
 	{ "/bin",          false },
 	{ "/boot",         true  },
 	{ "/cores",        false },
 	{ "/dev",          false },
 	{ "/etc",          false },
 	{ "/home",         true  },
-	{ "/Library",      false },
 	{ "/media",        true  },
 	{ "/mnt",          true  },
 	{ "/opt",          false },
@@ -50,7 +54,6 @@ const struct fhs_node fhs_root_nodes[] = {
 	{ "/tmp",          false },
 	{ "/usr",          false },
 	{ "/var",          false },
-	{ "/Volumes",      false },
 };
 
 const size_t fhs_root_node_count =

tests/test_visibility.c

@@ -84,6 +84,11 @@ main(void)
 	check("Linux-only nodes are marked",     fhs_node_find("/media")->linux_only);
 	check("native nodes are not",            !fhs_node_find("/usr")->linux_only);
 
+	/* macOS-only directories with no FHS counterpart are not presented. */
+	check("/Applications is not a node",     fhs_node_find("/Applications") == NULL);
+	check("/Library is not a node",          fhs_node_find("/Library") == NULL);
+	check("/Volumes is not a node",          fhs_node_find("/Volumes") == NULL);
+
 	/* Every lock value must have a reason; only CHANGEABLE has none. */
 	check("CHANGEABLE has no reason",   fhs_vis_lock_reason(FHS_VIS_CHANGEABLE) == NULL);
 	check("ABSENT has a reason",        fhs_vis_lock_reason(FHS_VIS_ABSENT) != NULL);
@@ -164,6 +169,8 @@ main(void)
 	rmdir(TEST_SCRATCH);
 
 	/* --- classification of the real root, read-only --------------------- */
+	static const char *const changeable[] = { "/opt", "/cores", "/Volumes" };
+
 	printf("\n  (the following read real system paths; none are modified)\n");
 
 	check_lock("/bin",     FHS_VIS_SIP,         "SIP");
@@ -182,9 +189,22 @@ main(void)
 		check_lock("/home", FHS_VIS_ABSENT, "absent");
 	check_lock("/private", FHS_VIS_PROTECTED,   "protected");
 	check_lock("/dev",     FHS_VIS_UNSUPPORTED, "unsupported");
-	check_lock("/opt",     FHS_VIS_CHANGEABLE,  "changeable");
-	check_lock("/cores",   FHS_VIS_CHANGEABLE,  "changeable");
-	check_lock("/Volumes", FHS_VIS_CHANGEABLE,  "changeable");
+	/*
+	 * The changeable nodes, asserted only where they exist.
+	 *
+	 * /opt is the instructive one: it is not part of a stock macOS install at
+	 * all - Homebrew creates it - so hard-coding "exists and is changeable"
+	 * makes the test suite fail on a machine without Homebrew, for a reason
+	 * that has nothing to do with this code. The property worth pinning is
+	 * that a node which *is* present classifies as measured, so an OS change
+	 * is still caught.
+	 */
+	for (size_t i = 0; i < sizeof(changeable) / sizeof(changeable[0]); i++) {
+		if (fhs_vis_status(changeable[i], &st) == 0 && st.exists)
+			check_lock(changeable[i], FHS_VIS_CHANGEABLE, "changeable");
+		else
+			check_lock(changeable[i], FHS_VIS_ABSENT, "absent");
+	}
 
 	/* /dev is the case the verification logic exists for. */
 	check("/dev is a mount point",