backup deletedump: Change -port to -portoffset

Use -portoffset instead of -port, for consistency with all of the
other backup suite commands. Leave -port in as an alias, for backwards
compatibility. Currently -port will mean -portoffset anyway, since
it's an unambiguous abbreviation, but put in the alias explicitly,
just in case some other option comes along starting with -port.

Change-Id: I2f8aaa34fdf9e7c80a8fec1dc1caf63d9b7192b1
Reviewed-on: http://gerrit.openafs.org/10440
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>