pginstaller - pginst: Add a check to ensure that the server encoding

Log Message:
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Add a check to ensure that the server encoding selected is valid for the locale. We allow the default encoding for the locale (eg. whatever Windows uses if setlocale(LC_CTYPE, "English_United Kingdom") is called or SQL_ASCII. UTF-8 should also be accepted, pending server changes currently being discussed on pgsql-hackers. If C is chosen as the locale, we accept any server encoding.

Also, ensure that the default server encoding offered is actually valid, otherwise default back to SQL_ASCII.

Modified Files:
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pginst/ca:
locale.c (r1.18 -> r1.19)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pginstaller/pginst/ca/locale.c.diff?r1=1.18&r2=1.19)
pginstca.c (r1.115 -> r1.116)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pginstaller/pginst/ca/pginstca.c.diff?r1=1.115&r2=1.116)