langinfo-h, nl_langinfo: Support abbreviated alternative month names. * lib/langinfo.in.h (ABALTMON_1...ABALTMON_12): New macros. * m4/langinfo_h.m4 (gl_LANGINFO_H): Define HAVE_LANGINFO_ABALTMON. * modules/langinfo-h (Makefile.am): Substitute HAVE_LANGINFO_ABALTMON. * tests/test-langinfo-h.c (items): Add ABALTMON_1 to ABALTMON_12. * lib/nl_langinfo.c (rpl_nl_langinfo): Treat ABALTMON_i like ABMON_i. * tests/test-nl_langinfo.c (main): Test ABALTMON_*. * doc/posix-headers/langinfo.texi: Document support of ABALTMON_*. * doc/posix-functions/nl_langinfo.texi: Likewise.