Use only one version of WritePidFile

This patch merges the pid file handling used for ganeti-* daemons and
impexp daemons. The latter version is used, since it's more reliable:
uses locked pid files as opposed to checking 'live' processes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>