Detect busybox logger and change logger cmdline

The busybox logger command (used on embedded systems and such) does not
support the -i option and die squaking if it's used. If the logger
configured is a symlink to busybox, the -i option must not be used.
Changed backup_script destination param to be optionnal.
rsnapshot-diff: Fixed removed files reported as addition (+ mark)

Fix an obvious copy/paste mistake in rsnapshot-diff.pl introduced
by 00e2f5085bd476ab867595d0ecbbed71264e9b5f
Fixed pod2man location.

Uses the pod2man present in the user path. Especially usefull if the
user has perl & pod2man installed in /usr/local/
Fixed SSH args after rsync command rollback

The rollback introduced in c595c567ee1a8ed928aadd6e47cf30af4e8f2e09
(related to changes from 57f7d836e53c3d47762988007b9627005b70fe4f) also
needs to rollback 5af353fa146303efe153a4db2ce5e3506c442123.