pgsql: Make libpq_gettext save and restore errno in a Windows-compatible

Log Message:
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Make libpq_gettext save and restore errno in a Windows-compatible way.
Also, back-patch fix into back branches.

Modified Files:
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pgsql/src/interfaces/libpq:
fe-misc.c (r1.115 -> r1.116)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-misc.c.diff?r1=1.115&r2=1.116)