Minor code refactoring in elog.c (no functional change).

Minor code refactoring in elog.c (no functional change).

Combine some duplicated code stanzas by creating small functions.
Most of these duplications arose at a time when I wouldn't have
trusted C compilers to auto-inline small functions intelligently,
but they're probably poor practice now.  Similarly split out some
bits that aren't actually duplicative as the code stands, but would
become so after an upcoming patch to add another error-handling
code path.

Take the opportunity to add some lengthier comments about what
we're doing here, too.  Re-order one function that seemed not
very well-placed.

Patch by me, per suggestions from Andres Freund.

Discussion: https://postgr.es/m/3bbbb0df-7382-bf87-9737-340ba096e034@postgrespro.ru