Previously, if no host information had been specified at connection time,
PQhost() would return NULL (unless you are on Windows, in which case you
got "localhost").  This is an unhelpful definition for a couple of reasons:
it can cause corner-case crashes in applications (cf commit c5ef8ce5),
and there's no well-defined way for applications to find out the socket
directory path that's actually in use.  As an example of the latter
problem, psql substituted DEFAULT_PGSOCKET_DIR for NULL in a couple of
places, but this is subtly wrong because it's conceivable that psql is
using a libpq shared library that was built with a different setting.

Hence, change PQhost() to return DEFAULT_PGSOCKET_DIR when appropriate,
and strip out the now-dead substitutions in psql.  (There is still one
remaining reference to DEFAULT_PGSOCKET_DIR in psql, in prompt.c, which
I don't see a nice way to get rid of.  But it only controls a prompt
abbreviation decision, so it seems noncritical.)

Also update the docs for PQhost, which had never previously mentioned
the possibility of a socket directory path being returned.  In passing
fix the outright-incorrect code comment about PGconn.pgunixsocket.