pgsql: Remove croak and Perl_croak from gettext triggers.

Log Message:
-----------
Remove croak and Perl_croak from gettext triggers.  While we could
selectively mark up their arguments for translation, the Perl xsubpp tool
generates a bunch of additional Perl_croak calls that we cannot control,
so we'd be creating a confusing mix of translated and untranslated messages
of a similar kind.  This is something that might deserve a more
comprehensive solution later.

Also remove _ from gettext triggers, because it wasn't used.

Use SPI.c instead of SPI.xs as source file for xgettext, because the .xs
format isn't really supported in xgettext.

Modified Files:
--------------
    pgsql/src/pl/plperl:
        nls.mk (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/nls.mk?r1=1.2&r2=1.3)