GUC time unit spelling a bit inconsistent

It seems that time-based GUC variables can be spelled like
	1h	but not		1hr
	1min	but not		1m
	1s	but not		1sec
This is inconsistent and confusing.  I don't object to the ones on the
left as being the standard spellings for printout, but if we're not
going to have a simple uniform rule like "shortest possible
abbreviation" then we ought to accept plausible alternatives on input.

I got burnt by this just now because I looked at the autovacuum_naptime
setting in postgresql.conf, which is shown as '1min', and figured I
could change it to '5sec'.

			regards, tom lane