Remove <DAEMON>_PID constants

The <DAEMON>_PID constants were created to reference a daemon pid file,
but actually contain a daemon's name, because the various functions that
work with pidfiles abstract the filename from the daemon name
themselves. Removing the constants and using the actual daemon name
constants in their place.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>