The current /etc/dumpdates file restricts device names to 32 characters.

The current /etc/dumpdates file restricts device names to 32 characters.
With the addition of various GEOM layers some device names now exceed
this length, for example /dev/mirror/encrypted.elig.journal. This
change expands the field to 53 bytes which brings the /etc/dumpdates
lines to 80 characters. Exceeding 80 characters makes the /etc/dumpdates
file much less human readable. A test is added to dump so that it
verifies that the device name will fit in the 53 character field
failing the dump if it is too long.

This change has been checked to verify that its /etc/dumpdates file
is compatible with older versions of dump.

Reported by: Martin Sugioarto <martin@sugioarto.com>
PR:          kern/160678
MFC after:   3 weeks