libexport.a: Add helpers to manage DNS lookups

libexport.a: Add helpers to manage DNS lookups

Introduce DNS query helpers based on getaddrinfo(3) and
getnameinfo(3).  These will eventually replace the existing
hostent-based functions in support/export/hostname.c.

Put some of these new helpers to immediate use, where convenient.

As they are part of libexport.a, I've added the forward declarations
for these new functions in exportfs.h rather than misc.h, where the
hostent-based forward declarations are currently.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>