usr.sbin/mkfs: Allow -b SIZE with proto file
usr.sbin/mkfs: Allow -b SIZE with proto file

Previously, if a proto file was given, the -b argument would be ignored.

Also added:
 - if the number of inodes is not given, then an estimation is used.
 - sanity checks where moved from one special case to the general case.

Change-Id: I2abfa52bf34206c9087c64b5bfc26af866eb47cb

veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi