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

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(dot)GA22965(at)postgresql(dot)kr>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9901d8ac2e7326f5a705341d304e7c7f0f95a1e5

Modified Files
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src/bin/psql/command.c | 25 +++++++++++++++----------
1 file changed, 15 insertions(+), 10 deletions(-)