pgsql: Make the numeric-OID cases of regprocin and friends be non-throw

Make the numeric-OID cases of regprocin and friends be non-throwing.

While at it, use a common subroutine already.

This doesn't move the needle very far in terms of making these
functions non-throwing; the only case we're now able to trap is
numeric-OID-is-out-of-range.  Still, it seems like a pretty
non-controversial step in that direction.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/780ec9f1b2a44c118d1246325404ad0ed2226cbf

Modified Files
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src/backend/utils/adt/regproc.c | 216 ++++++++++++++--------------------------
1 file changed, 77 insertions(+), 139 deletions(-)