Please test trace mode on El Capitan

[Clemens Lang]

Hi developers,

if you are running MacPorts trunk on El Capitan, please make sure you're at the
latest version and test trace mode. Originally, El Capitan's system integrity
protection broke trace mode because all DYLD_* variables are stripped when
executing a binary under SIP.

I modified our trace mode code to predict this situation, make a copy of the
binary when necessary (stripping the SIP flag) and transparently run the copy
instead of the original to avoid this situation.

We may want to backport this change into another 2.3.x bugfix release, so I'd
like to see more widespread testing first to clear out any remaining issues.

-- 
Clemens Lang