add phlsys_pid for working with process ids

Add convenient functions for getting the current pid, checking if a pid
is running and asking a pid to terminate.

Test Plan:
$ ./precommit.sh

Exercised by subsequent commits.
Arcyd: add abdt_fs for working with the filesystem

Centralise and codify the layout and naming of files in the arcyd
working directory.

Provide a class to attach to the directory and access it.

Test Plan:
$ ./precommit.sh

Exercised by subsequent commits.
Arcyd: start and stop commands use pids

Make Arcyd start and stop able to tell if an instance is already
running and potentially kill it by using it's pid.

Test Plan:
$ ./precommit.sh

start and stop with
$ ./testbed/arcyd/test_shell.sh