the following patch makes the filename used to store the readline

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