Fix reconnecting to pgsql database after connection loss.

Fix reconnecting to pgsql database after connection loss.

There can only be one database connection in res_config_pgsql just like
res_config_sqlite.  If the connection is lost, the connection may not get
reestablished to the same database if the res_pgsql.conf and
extconfig.conf files are inconsistent.

* Made only use the configured database from res_pgsql.conf.

* Fixed potential buffer overwrite of last[] in config_pgsql().

(closes issue ASTERISK-16982)
Reported by: german aracil boned

Review: https://reviewboard.asterisk.org/r/1731/
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Merged revisions 354953 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@354959 65c4cc65-6c06-0410-ace0-fbb531ad65f3