POSIX was out of whack with Windows as a result of the changes made
around channels. The schedular in posix was very different, and this
commit brings it into line.
Other than the obvious issues, a non-obvious issue with the changes
was that the channel was being freed up on close prior to the thread
terminating. This doesn't appear to be an issue on Windows, but was
causing crashes on close in POSIX.
The changes go quite deep. This changeset requires a lot of testing.
The goals of this work are:
* To fix issue where backgrounding and re-interacting with channels wasn't
working.
* To fix issue where closing of meterpreter was not closing off background
prcoesses (such as cmd.exe).
The two things preventing this stuff from working were:
* When interactive channels are backgrounded their handles were destroyed
along with the context that wraps them up. Making them interactive again
had no impact because the handle and context were invalid. If anything,
this made meterpreter unstable. Sometimes the session would die when
attempting to interact with the channel again.
* When closing channels, there was no way of terminating the process that
sat behind the scenes because no reference to the process was retained.
Channels would close and handles would close, but no process termination
was done.
To fix these problems:
* The interactive thread no longer terminates when backgrounded. Instead
its put in a suspended state where it's waiting a signal from a resume
handle that's associated with the channel's context. This means that the
destruction of the context doesn't happen at all until the termination
of the channel, which is exactly when it should happen anyway.
* Process handles are stored alongside the input/output handles so that
when the time comes, the process can be terminated if required. This
means that when the channels are closed, the code has a reference to the
associated process which can be terminated. This is only done for
interactive processes, non-interactive processes do not have this
problem because meterpreter doesn't have to keep track of them.
