prussdrv: Remove unnecessary pthreads dependency

This removes the helper function for creating a pthread and the rest of
the infrastructure around it. Now the library can be linked in
environments that don't have pthreads. Also, the pthread helper code is
easily replicated in a user application and probably makes more sense
there anyway so that the user is more aware of the threading.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
PRU updates from the Linux CNC project

This only contains a few of the updates. The Linux CNC project has a
good refactoring of the internal data structure that may be nice to pull
in as well. Also left out are the debug logging updates.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
prussdrv: add call to get file descriptor

This is needed to be able to select(2) or poll(2) on PRU interrupts.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>