New mergemaster option -I, failsafe install files

Hi Doug,

As you may guess from my multiple emails, I'm in the process of
upgrading my jails :-).

Since I have one jail per service, a very few number of configuration
files are modified on each jail.  As most of user of FreeBSD I think,
I'm used to run "mergemaster -iU" to automate the process as much as
possible.  The problem with service jails (chapter 15.6.1 of the
handbook) is that / is read-only mounted on all jails, /etc /var /root
and a few other places being symlinks to /s, the private read-write
space of each jail.  Thus when mergemaster tries to update
/boot/devices.hints it fails and abort.

Therefore I've implemented a new -I option that does the same thing as
-i except that it will proceed on failure.  At the end of the process,
it will also show an informational message about files that could not be
installed.

This patch along with the fix I sent you a few hours ago allows to use
mergemaster(8) in service jails flawlessly.

Best regards,
-- 
Jeremie Le Hen
< jeremie at le-hen dot org >< ttz at chchile dot org >