vkernel - enhance the pidfile option and fix memimg file scanning

vkernel - enhance the pidfile option and fix memimg file scanning

* Enhance the pidfile option to leave the file descriptor open and hold
  an active lock on the pidfile while the vkernel is running.

  This allows scripts to test whether the pidfile is real or stale.

* When scanning for memimg files only stop if we are unable to create
  a new file.  Skip files owned by other users instead of giving up.

* Adjust the vkernel manual page to document the pidfile feature.  Also
  document that -I can take a vknetd socket path.