Print rnspashot's PID when logging to syslog (instead of the logger's program PID)

This is much more useful because it allow to relate log lines to rsnapshot instances when there are multiple concurrent ones. Previously each logged line had a different PID, even those generated by the same rsnapshot instance.

This is achieved by removing the "-i" option from the logger's call, and appending the program's pid to the syslog tag (-t option) directly. Coincidentally, this obsoletes commit 0a056977d736df3adcc16e99a50dd06efbdb7a0e (which removed the "-i" option when the busybox logger was detected), so this commit is reverted too.