Improve growfs(8) in a few ways; unfortunately, it's somewhat hard to untangle

Improve growfs(8) in a few ways; unfortunately, it's somewhat hard to untangle
them and commit separately.

1. Rewrite the way growfs(8) finds the device and mount point.  This makes
   it possible to use e.g. "growfs /mnt"; it's also used to display more
   helpful messages.

2. Be more user-friendly, using descriptive messages, like this:

   OK to grow filesystem on /dev/md0, mounted on /mnt, from 9.8GB to 20GB? [Yes/No]"

3. Allow to specify the size (-s option) just like with mdconfig(8), i.e. with
   postfixes ("mdconfig -s 10g").

4. Reload read-only filesystem after growing.

Reviewed by:	kib, mckusick (earlier version)
Sponsored by:	The FreeBSD Foundation