Add GUC variables to control keep-alive times for idle, interval, and

doc/src/sgml/runtime.sgml

src/backend/libpq/pqcomm.c

src/backend/utils/misc/guc.c

src/backend/utils/misc/postgresql.conf.sample

src/bin/psql/tab-complete.c

src/include/libpq/libpq-be.h

src/include/utils/guc.h