pgsql: Clean up a number of bogosities around pltcl's handling of the

Log Message:
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Clean up a number of bogosities around pltcl's handling of the Tcl "result":

1. Directly reading interp->result is deprecated in Tcl 8.0 and later;
you're supposed to use Tcl_GetStringResult. This code finally broke with
Tcl 8.5, because Tcl_GetVar can now have side-effects on interp->result even
though it preserves the logical state of the result. (There's arguably a
Tcl issue here, because Tcl_GetVar could invalidate the pointer result of a
just-preceding Tcl_GetStringResult, but I doubt the Tcl guys will see it as
a bug.)

2. We were being sloppy about the encoding of the result: some places would
push database-encoding data into the Tcl result, which should not happen,
and we were assuming that any error result coming back from Tcl was in the
database encoding, which is not a good assumption.

3. There were a lot of calls of Tcl_SetResult that uselessly specified
TCL_VOLATILE for constant strings. This is only a minor performance issue,
but I fixed it in passing since I had to look at all the calls anyway.

#2 is a live bug regardless of which Tcl version you are interested in,
so back-patch even to branches that are unlikely to be used with Tcl 8.5.
I went back as far as 8.0, which is as far as the patch applied easily;
7.4 was using a different error processing scheme that has got its own
problems :-(

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/src/pl/tcl:
pltcl.c (r1.108 -> r1.108.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/tcl/pltcl.c?r1=1.108&r2=1.108.2.1)