pgsql: the following patch makes the filename used to store the readline

Log Message:
-----------
the following patch makes the filename used to store the readline
history customizable through a variable named HISTFILE, analogous to
psql's already implemented HISTCONTROL and HISTSIZE variables, and
bash's HISTFILE-Variable.

The motivation was to be able to get psql to maintain separate
histories for separate databases. This is now easily achievable
through a line like the following in ~/.psqlrc:

\set HISTFILE ~/.psql_history-:DBNAME

Andreas Seltenreich

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
psql-ref.sgml (r1.139 -> r1.140)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/psql-ref.sgml.diff?r1=1.139&r2=1.140)
pgsql/src/bin/psql:
input.c (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/input.c.diff?r1=1.43&r2=1.44)