unsetenv etc.: port to Solaris 11 + GNU Emacs

unsetenv etc.: port to Solaris 11 + GNU Emacs

* lib/canonicalize-lgpl.c, lib/getaddrinfo.c, lib/getdelim.c:
* lib/glob.c, lib/random_r.c, lib/setenv.c, lib/tsearch.c:
* lib/unsetenv.c (_GL_ARG_NONNULL): Define before including <config.h>.
GNU Emacs's <config.h> includes <stdlib.h> (which is not a great
idea but is too painful to fix right now), and without this gnulib
change <stdlib.h> was defining _GL_ARG_NONNULL incorrectly when
compiling unsetenv.c on Solaris 11.  Fix the problem for
unsetenv.c, and fix other similar occurrences.