Improving trace mode

[Rainer Müller]

Hello,

Trace mode (port -t) is a very good way to find out which files are
accessed and need to be available during building a port. But currently
this is not fully usable, as it has some problems with the way it
currently works.

When a file access outside of the sandbox (deps, Xcode, SDKs etc.) is
detected, it denies access to this file. But if you build some port and
it cannot access a file the build often either fails completely or is
unfinished.

In order to improve this, I would like to change the behavior of trace
mode. In my opinion it would be sufficient to report access to files
outside of the sandbox only and let the maintainer decide if these are
valid or not. Denying access in trace mode makes the build behave
differently as if it is run without trace mode.

Although I don't think the current behavior is really useful, we could
keep it and as an alternative add a new switch (port -T?) to implement
the behavior I am asking for.

Rainer