pgsql: Produce a more useful error message for over-length Unix socket

Produce a more useful error message for over-length Unix socket paths.

The length of a socket path name is constrained by the size of struct
sockaddr_un, and there's not a lot we can do about it since that is a
kernel API. However, it would be a good thing if we produced an
intelligible error message when the user specifies a socket path that's too
long --- and getaddrinfo's standard API is too impoverished to do this in
the natural way. So insert explicit tests at the places where we construct
a socket path name. Now you'll get an error that makes sense and even
tells you what the limit is, rather than something generic like
"Non-recoverable failure in name resolution".

Per trouble report from Jeremy Drake and a fix idea from Andrew Dunstan.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/31c341ae13eb8adbc0007baa303efd84c2735fbe

Modified Files
--------------
src/backend/libpq/pqcomm.c | 8 ++++++++
src/include/libpq/pqcomm.h | 13 +++++++++++++
src/interfaces/libpq/fe-connect.c | 11 ++++++++++-
3 files changed, 31 insertions(+), 1 deletions(-)