I previously thought it wouldn't be possible to probe arbitrary user-mode addresses,
but it turns out that it is. This was required to support global variable arguments
in USDT probes. For example, there are a few probes in libpthread whose arguments
refer to the mp_ global variable. These are now supported.

Additionally, refactored process-related utils into a ProcUtils class in procstat.py,
and implemented a version of `which` (`ProcUtils.which`) that is as portable as
possible -- it only relies on traversing the PATH environment variable.