Cleanup xmalloc.

Cleanup xmalloc.
* admin/coccinelle/xzalloc.cocci: Semantic patch to convert
calls to xmalloc with following memset to xzalloc.
* src/lisp.h (xzalloc): New prototype.  Omit needless casts.
* src/alloc.c (xzalloc): New function.  Omit needless casts.
* src/charset.c: Omit needless casts.  Convert all calls to
malloc with following memset to xzalloc.
* src/dispnew.c: Likewise.
* src/fringe.c: Likewise.
* src/image.c: Likewise.
* src/sound.c: Likewise.
* src/term.c: Likewise.
* src/w32fns.c: Likewise.
* src/w32font.c: Likewise.
* src/w32term.c: Likewise.
* src/xfaces.c: Likewise.
* src/xfns.c: Likewise.
* src/xterm.c: Likewise.
* src/atimer.c: Omit needless casts.
* src/buffer.c: Likewise.
* src/callproc.c: Likewise.
* src/ccl.c: Likewise.
* src/coding.c: Likewise.
* src/composite.c: Likewise.
* src/doc.c: Likewise.
* src/doprnt.c: Likewise.
* src/editfns.c: Likewise.
* src/emacs.c: Likewise.
* src/eval.c: Likewise.
* src/filelock.c: Likewise.
* src/fns.c: Likewise.
* src/gtkutil.c: Likewise.
* src/keyboard.c: Likewise.
* src/lisp.h: Likewise.
* src/lread.c: Likewise.
* src/minibuf.c: Likewise.
* src/msdos.c: Likewise.
* src/print.c: Likewise.
* src/process.c: Likewise.
* src/region-cache.c: Likewise.
* src/search.c: Likewise.
* src/sysdep.c: Likewise.
* src/termcap.c: Likewise.
* src/terminal.c: Likewise.
* src/tparam.c: Likewise.
* src/w16select.c: Likewise.
* src/w32.c: Likewise.
* src/w32reg.c: Likewise.
* src/w32select.c: Likewise.
* src/w32uniscribe.c: Likewise.
* src/widget.c: Likewise.
* src/xdisp.c: Likewise.
* src/xmenu.c: Likewise.
* src/xrdb.c: Likewise.
* src/xselect.c: Likewise.