Add NI_NUMERICSCOPE.

POSIX also declares NI_NUMERICSCOPE, which makes getnameinfo() return a
numerical scope identifier. The interesting thing is that support for
this is already present in code, but #ifdef disabled. Expose this
functionality by placing a definition for it in <netdb.h>.

While there, remove references to NI_WITHSCOPEID, as that got removed 11
years ago.