Add PF_LOCAL support in getaddrinfo(3) and getnameinfo(3):

• In a PF_LOCAL address, "hostname" must begins with '/' and "servname" is always NULL. All of ai_flags are ignored.

• PF_UNSPEC matches PF_LOCAL. EAI_SERVICE is not returned to make AF-independent programming easier; "servname" is always ignored in PF_LOCAL. In practice, PF_INET* and PF_LOCAL are mutually-exclusive because a hostname which begins with '/' is invalid in PF_INET*. No domain name resolution is performed for a PF_LOCAL address.

Differential Revision: https://reviews.freebsd.org/D3634