Backup hooks in dry-run mode

[ewxrjk]

As the man page says:

Backup hooks are currently not executed in --dry-run mode but note that this
will be changed in the future and an RSBACKUP_ACT variable introduced, as
for access hooks.

'The future' will mean release 2.0 (on the grounds that 1.0 shouldn't contain any big surprises for existing users).