mpool/udreg: minor comment improvement
Two cleanups required for transfer to 1.8.4:

* Use %d format for the topo signature as some systems apparently have problems with %u
* Use correct variable in show_help message
Correct some issues with variables used before being set
It looks like the guess_len function in our local printf.c has some questionable code in it. Now that we are checking in configure for vsnprintf, take advantage of that check to use the far simpler method if it is available. Given that we no longer support such ancient systems where this might not be available, one suspects the other questionable code may no longer be required - but set that aside for another day.