Generalize run-time debugging checks.

Generalize run-time debugging checks.
* configure.in (ENABLE_CHECKING): Update comment.
* src/dispextern.h (XASSERTS): Remove.
* src/fontset.c (xassert): Remove.
Convert from xassert to eassert.
* src/alloc.c: Convert from xassert to eassert.
* src/bidi.c: Likewise.
* src/dispnew.c: Likewise.
* src/fns.c: Likewise.
* src/fringe.c: Likewise.
* src/ftfont.c: Likewise.
* src/gtkutil.c: Likewise.
* src/image.c: Likewise.
* src/keyboard.c: Likewise.
* src/menu.c: Likewise.
* src/process.c: Likewise.
* src/scroll.c: Likewise.
* src/sound.c: Likewise.
* src/term.c: Likewise.
* src/w32console.c: Likewise.
* src/w32fns.c: Likewise.
* src/w32term.c: Likewise.
* src/window.c: Likewise.
* src/xdisp.c: Likewise.
* src/xfaces.c: Likewise.
* src/xfns.c: Likewise.
* src/xselect.c: Likewise.
* src/xterm.c: Likewise.