ionice: add -t option

ionice: add -t option

This patch allows "tolerant" behavior, i.e. proceeding even if
priority could not be set. This might be of use in case something
(selinux, old kernel, etc.) does not allow the requested scheduling
priority to be set.

This could be to some extend done as follows:

	ionice -c3 command || command

but the downside is that one could not really tell if what failed was
setting priority or command itself, which could result in duplicate
command run.

This patch solves the situation, so that user can do

	ionice -t -c3 command

Addresses-Red-Hat-Bugzilla: #443842
Signed-off-by: Lubomir Kundrak <lkundrak@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>