Fix compiler warnings

[roblillack]

The build produced >100 warnings across the tree, dominated by undefined-behavior
casts, K&R-style function declarations, and a deprecated X11 API. This PR fixed that:

Pointer/int casts (-Wpointer-to-int-cast, -Wvoid-pointer-to-int-cast)

• FSBrowser.c, FSPathView.c: drop bogus assert((int)ptr) casts that
  truncate on 64-bit; the assertion only needs the pointer.
• FSBrowser.c, FSFinder.c, FSPanel.c, extnInspector.c: route
  WMGetNotificationClientData(...) through intptr_t before narrowing to
  int.

Switch coverage (-Wswitch)

• FSBrowser.c, FSPathView.c: add WSDecrementWheel / WSIncrementWheel
  cases (no-op, matching the existing WSKnobSlot / WSNoPart arm).
• FSFileView.c: handle FileLink in FSUpdateFileViewPath; treated like
  FileCopy since it introduces a new entry into the view.

Missing prototypes (-Wdeprecated-non-prototype)

• FSUtils.h: give FSUpdateFileView a real prototype (was ()).
• magic.c, regexp/regexp.c, regexp/regsub.c, regexp/regerror.c,
  regexp/regexp.h: convert all K&R declarations and definitions to ANSI
  prototypes. Pure mechanical rewrite; no behavior change.

Misc

• FSFileView.c: drop extraneous parens in if ((x == y))
  (-Wparentheses-equality).
• FSUtils.c: replace WMColor* with an RColor literal in the call to
  WMCreateBlendedPixmapFromFile, which actually takes const RColor*
  (-Wincompatible-pointer-types).
• FSPanel.c: replace if (txt != ICONDIR) pointer-literal compare with an
  explicit "allocated" flag (-Wstring-compare).
• xmodifier.c: replace deprecated XKeycodeToKeysym with
  XkbKeycodeToKeysym (-Wdeprecated-declarations).