statd: add nsm_present_address() API

Add an API to convert a socket address to a presentation address
string.  This is used for displaying error messages and the like.

We prefer getnameinfo(3) over inet_?to?(3) as it supports IPv6 scope
IDs.  Since statd has to continue to build correctly on systems whose
glibc does not have getnameinfo(3), an inet_?to?(3) version is also
provided.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>