dumpon: accept "dump" type in fstab

[emaste]

PR: 175311

In contrast to the approach in the PR I went with a loop looking for dump first then swap, as dump should take precedence over swap, in e.g.

# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/nvd0p3             none    swap    sw              0       0
/dev/nvd0p4             none    dump    sw              0       0

[emaste]

or if we want to avoid reading /etc/fstab twice something like

dump_try=
swap_try=
while read dev mp type more ; do
        [ "${type}" = dump -o "${type}" = swap ] || continue
        case ${dev} in
        *.bde|*.eli)
                dumpon_warn_unencrypted
                dev=${dev%.*}
                ;;
        esac
        [ -c "${dev}" ] || continue
        eval ${type}_try=\"\${${type}_try} ${dev}\"
done </etc/fstab
for dev in ${dump_try} ${swap_try}; do
        dumpon_try "${dev}" 2>/dev/null && return 0
done