Fix failure to remove non-first segments of temporary tables.

Commit 4ab5dae94 broke mdunlinkfork's logic for removing additional
segments of a multi-gigabyte table, because it neglected to advance
"segno" after unlinking the first segment, in the code path where it
chooses to unlink that one immediately.  Then the main remove loop
gets ENOENT at segment zero and figures it's done, so we never remove
whatever additional segments might exist.

The main problem here is with large temporary tables, but WAL replay
of a drop of a large regular table would also fail to remove extra
segments.  The third case where this path is taken is for non-main
forks; but I doubt it matters for those since they probably never
exceed 1GB.

The simplest fix is just to increment segno after that unlink().
(Probably this logic could do with a more thorough rethink, but not
with mere hours to go before 15.1 wraps.)

While here, also fix an incautious assumption that
register_forget_request cannot change errno.  I don't think that
that has any really bad consequences, as we'd end up trying to unlink
the zero'th segment either way, but it greatly complicates reasoning
about what could happen here.  Also make a couple of other cosmetic
fixes.

Per bug #17679 from Balazs Szilfai.  Back-patch into v15, as the
faulty patch was.

Discussion: https://postgr.es/m/17679-1095d04450cf6a6e@postgresql.org

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