rewrite parts of the Makefile in setup.py. Add 'build_tahoe' and 'trial' subcommands.

rewrite parts of the Makefile in setup.py. Add 'build_tahoe' and 'trial' subcommands.

The 'make build' target now runs 'setup.py build_tahoe', which figures out
where the target 'supportlib' directory should go, and invokes 'setup.py
develop' with the appropriate arguments.

The 'make test' target now runs 'setup.py trial', which manages sys.path and
runs trial as a subroutine instead of spawning an external process. This
simplifies the case where Twisted was built as a dependent library (and thus
the 'trial' executable is not on PATH).

setup.py now manages sys.path and PYTHONPATH for its internal subcommands, so
the $(PP) prefix was removed from all Makefile targets that invoke setup.py .
For the remaining ones, the 'setup.py -q show_pythonpath' subcommand was
added to compute this prefix with python rather than with fragile
shell/Makefile syntax.