pgsql: Add \ir command to psql.

Add \ir command to psql.

\ir is short for "include relative"; when used from a script, the
supplied pathname will be interpreted relative to the input file,
rather than to the current working directory.

Gurjeet Singh, reviewed by Josh Kupershmidt, with substantial further
cleanup by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/c7f23494c1103f87bcf1ef7cbfcd626e73edb337

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml |   15 +++++++++++++++
src/bin/psql/command.c         |   37 ++++++++++++++++++++++++++++++++-----
src/bin/psql/command.h         |    2 +-
src/bin/psql/help.c            |    3 ++-
src/bin/psql/settings.h        |    2 +-
src/bin/psql/startup.c         |    6 +++---
src/bin/psql/tab-complete.c    |    3 ++-
src/include/port.h             |    1 +
src/port/path.c                |   11 +++++++++++
9 files changed, 68 insertions(+), 12 deletions(-)