squidpurge: polish debug -d option handling

squidpurge: polish debug -d option handling

Parsing of the -d option parameter was severaly broken with an inverted
NULL check resulting in segfaul whenever a level was supplied.

Also, add validation that out-of-range values cannot be supplied and
document in the help that this is a mask value, not just a number.

 Detected by Coverity Scan. Issue 740378.