Do not use a predictible filename in /tmp for pid

Since any user can write the file in /tmp, any user could inject a
incorrect pid or anything ( that would be passed to kill
later ), or to put a link to a file ( which would be erased if the timing is
good ) when SparkleShare is started. So using a file in the hme, prefixed
with the hostname ( for nfs shared home ). This is a problem only on
distribution where $XDG_RUNTIME_DIR is not set ( for example RHEL 6 ).