procfs: convert to KNF

Change-Id: Ib4252f199af0f9597745dcd2c11a7f761738671f
procfs: add /proc/service directory

This directory is filled dynamically with regular files, one for each
service that RS knows about, named after its label.  Its contents are
still subject to (heavy) change, but currently expose the service's
endpoint and number of restarts so far.

Change-Id: Ie58c824bcb6382c8da7a714e59fee87329970b4b
libfsdriver: prefill st_dev for stat requests

This obviates the need for several file system implementations to
remember the device on which they are mounted.

Change-Id: Ida8325cf4bcf072e61761cfee34e3f7ed2d750b9