Avoid some lines longer than 80 characters.

Avoid some lines longer than 80 characters.

* lib/stdint.in.h: Break long comment lines.
* lib/math.in.h: Likewise.
(_GL_NUM_UINT_WORDS): New macro, for readability.
(gl_signbitf, gl_signbitd, gl_signbitl): Use it.
* lib/stdio.in.h: Break lines in _GL_WARN_ON_USE calls.
* lib/stdlib.in.h: Likewise.
* lib/spawn.in.h: Likewise.
* lib/sys_socket.in.h: Update an URL.
* lib/sys_stat.in.h: Break long line.