Ensure filenames are comprised of 1 or more valid characters.

A filename may not be "".

Depending on the filesystem, not all filenames are acceptable. We
restrict ourselves to alphanumberics (with case) plus a few symbols.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge branch 'master' of git://github.com/xen-org/ffs
Update to 0.9.2.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>