Fix PGXS vpath build when PostgreSQL is built with vpath

PGXS computes srcdir from VPATH, PostgreSQL proper computes VPATH from
srcdir, and doing both results in an error from make.  Conditionalize so
only one of these takes effect.
Document that pg_stat_*_tables.n_tup_upd includes n_tup_hot_upd.

Author: Peter Geoghegan
Don't dump core if pq_comm_reset() is called before pq_init().

This can happen if an error occurs in a standalone backend.  This bug
was introduced by commit 2bd9e412f92bc6a68f3e8bcb18e04955cc35001d.

Reported by Álvaro Herrera.