Use buffered I/O by default in the command-line tool

This speeds up activities like using 'fat add' to generate
the mirage-www files.img from 4:30s to 0:01.8s on my laptop
(with spinning disks)

Unbuffered I/O can still be used by adding '--unbuffered'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>