by extending the ereport() API to cater for pluralization directly.  This
is better than the original method of calling ngettext outside the elog.c
code because (1) it avoids double translation, which wastes cycles and in
the worst case could give a wrong result; and (2) it avoids having to use
a different coding method in PL code than in the core backend.  The
client-side uses of ngettext are not touched since neither of these concerns
is very pressing in the client environment.  Per my proposal of yesterday.

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.

the ereport macro.  Included in this commit are enough files for starting
plpgsql, plpython, plperl and pltcl translations.