libz: add an io compartment

zlib has two major parts.  The core deflate compression works entierly
in memory.  There is also the gzlib portion which provides an analog to
POSIX IO interfaces and uses IO underneath.

Author: brooksdavis

lib/libz/Compartments.json

@@ -0,0 +1,9 @@
+{
+    "compartments": {
+	"io": {
+	    "files": [
+		"gz*.*o"
+            ]
+        }
+    }
+}

lib/libz/Makefile

@@ -51,6 +51,7 @@ CFLAGS+=	-DUNALIGNED_OK
 
 VERSION_DEF=	${.CURDIR}/Versions.def
 SYMBOL_MAPS=	${.CURDIR}/Symbol.map
+COMPARTMENT_POLICY=	${.CURDIR}/Compartments.json
 
 INCS=		zconf.h zlib.h