Add -L option to psql to log sessions.

doc/src/sgml/ref/psql-ref.sgml

src/bin/psql/common.c

src/bin/psql/describe.c

src/bin/psql/help.c

src/bin/psql/large_obj.c

src/bin/psql/print.c

src/bin/psql/print.h

src/bin/psql/settings.h

src/bin/psql/startup.c

src/bin/scripts/createlang.c

src/bin/scripts/droplang.c