Remove grep call for integer test; reject TTL of 0

A TTL of 0 is invalid IMO and does weird thing to seconds2TTL math.
Fix quoting bug in for-loop

I really /must/ stop creating this type of bug.
Remove unneeded global variables

And small comment fixes.
Merge branch 'small_fixes' of https://github.com/Alexqw/zfSnap into Alexqw-small_fixes