fuse2fs: add fakeroot option.

fuse2fs: add fakeroot option.

Add a new 'fakeroot' option to fuse2fs. When enabled, fuse2fs will
will pretend to be root when checking file permssions. This allows
fuse2fs to be used for building/modifying rootfs images as an
unprivileged user.

As per the maintainer's request, nosuid and nodev are automatically
enabled when fakeroot is selected (on platforms that support them)
to help prevent accidental misuse.

Signed-off-by: Nicholas Clark <nicholas.clark@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>