GUC time unit spelling a bit inconsistent

It seems that time-based GUC variables can be spelled like1h    but not        1hr1min    but not        1m1s    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