netdb-h: Ensure AI_NUMERICHOST and AI_NUMERICSERV are defined. * lib/netdb.in.h (AI_NUMERICHOST): New macro. * tests/test-netdb-h.c: Verify that AI_NUMERICHOST and AI_NUMERICSERV are defined. * doc/posix-headers/netdb.texi: Mention the platform support for AI_NUMERICHOST and AI_NUMERICSERV.