Move getaddrinfo.h declarations to netdb.h.

* lib/getaddrinfo.h: Remove file.
* modules/getaddrinfo: Reflect move from getaddrinfo.h to netdb.h.
* m4/getaddrinfo.m4: Call gl_HEADER_NETDB.  Don't check for netdb.h.
* lib/netdb.in.h: Add declarations from getaddrinfo.h.
* m4/netdb_h.m4: Initialize GNULIB_GETADDRINFO to 0.
* modules/netdb: Substitute GNULIB_GETADDRINFO.
* lib/getaddrinfo.c: Include netdb.h instead of getaddrinfo.h.
* tests/test-getaddrinfo.c: Likewise.
* lib/gai_strerror.c: Likewise.  Also drop HAVE_NETDB_H check.
* NEWS: Mention change.