Use strftime("%c") to format timestamps in psql's \watch command.

This allows the timestamps to follow local conventions (in particular,
they respond to the LC_TIME environment setting).  In C locale you get
the same results as before.  It seems like a good idea to do this now not
later because we already changed the format of \watch headers for 9.6.

Also, increase the buffer sizes a tad to ensure there's enough space for
translated strings.

Discussion: <20160612145532.GA22965@postgresql.kr>