Optimize pure C strings initialization.

Optimize pure C strings initialization.
* lisp.h (make_pure_string): Fix prototype.
(build_pure_c_string): New function, defined as static inline.  This
provides a better opportunity to optimize away calls to strlen when
the function is called with compile-time constant argument.
* alloc.c (make_pure_c_string): Fix comment.  Change to add nchars
argument, adjust users accordingly.  Use build_pure_c_string where
appropriate.
* buffer.c, coding.c, data.c, dbusbind.c, fileio.c, fontset.c, frame.c,
* keyboard.c, keymap.c, lread.c, search.c, syntax.c, w32fns.c, xdisp.c,
* xfaces.c, xfns.c, xterm.c: Use build_pure_c_string where appropriate.