getopt: clean up getopt.c and getopt1.c file headers
In getopt.c, there is no need to include wchar.h at all, and it is
safe nowadays to assume that stdlib.h does declare getenv (several
other gnulib modules make this assumption).
In getopt1.c, the #ifdef _LIBC block at the top can be simplified
by using "" inclusions consistently, and there is no actual need to
include stdlib.h (except in the #ifdef TEST block, where it should be
unconditional), nor to provide a backup definition of NULL at all.
* lib/getopt1.c: Simplify #ifdeffage at top of file.
Move inclusion of stdlib.h to #ifdef TEST block and make
unconditional. Do not define NULL.
* lib/getopt.c: Don't include wchar.h. No need to declare getenv.
* m4/getopt.m4 (gl_PREREQ_GETENV): Delete.
* modules/getopt-gnu, modules/getopt-posix: Don't call
gl_PREREQ_GETENV.