Reduce output noisyness

Do not print all the par2 files that have been deleted. This
drowns out the lines about the actual backup sets that were
removed.
Remove no-execute flag

Remove the no-execute flag from `zfs destroy'.
It is still set to verbose to print the reclaimed storage
size.
Fix badly broken storage cleanup filename regexp

* move the datepart into its own regexp
* reuse datepart regexp in file regexp
* precompile the regexp outside the loop
* gain readability
* add `-' as possible char in dataset names
* the type indicator postfix (FULL, DIFF, INCR)
  is all uppercase. And [A-z] a bug.
* fix date regexp value range:
  * no longer accept 5+ digit years. while not wrong,
    this is probably too optimistic
  * no longer accept months 00 and 13 to 19
  * no longer accept day 00
  * accept days 30 and 31, they are real
* skip files that do not match the regexp. This implicits:
  do not delete what we do not know about
* never go with a quick 'probably good-enough'
  regexp that you feel embarrased about later
* update README accordingly

The regexp still does no verification that the
day of the month actually exists in that month.
But handling that in regexp is, while certainly
possible, not making it any more readable.
I'm not in the mood to add a proper date library
there.