Allow to_date() and to_timestamp() to accept localized month names

Hi,
attached is a patch which adds support to localized month names in
to_date() and to_timestamp() functions.

The patch is fairly simple but I want to discuss the approach and
implementation:

Using the TM modifier as in to_char() was already discussed some years
ago: 10710.1202170898@sss.pgh.pa.us [1]

I thought about reusing from_char_seq_search() but localized month
names use different capitalization according to the language grammar,
so I used pg_strncasecmp to do the match.

Regression tests with TM modifier are difficult since one should have
the locale used for the test installed on his system.

Usage example:
postgres=# set lc_time to 'fr_FR';
SET
postgres=# select to_date('22 janvier 2016', 'DD TMMonth YYYY');
  to_date
------------
 2016-01-22
(1 row)

[1] https://www.postgresql.org/message-id/10710.1202170898%40sss.pgh.pa.us

Thanks
Mattia