WRKDIRPREFIX cleanup should be optional

[p]

I'm talking about this: https://github.com/p/portmaster/commit/c0e147a0c8fa26661a05cbd370e6fcbf24f0d2b3

On my systems this tends to make portmaster effectively hang for anywhere between 3 to 15 seconds after aborting installs/updates.

Considering that relatively large and possibly huge amounts of disk are required for building ports, any savings that can be obtained from deleting empty directories ought to be irrelevant. But it makes quitting (and therefore using) portmaster quite annoying.