Description of problem:

virt-make-fs fails if you are creating a VFAT format
filesystem, and any file has g+s or u+s bits set.

Version-Release number of selected component (if applicable):

libguestfs-1.29.48-1.fc23.x86_64

How reproducible:

100% - see below.

Steps to Reproduce:
1. mkdir /tmp/p
2. touch /tmp/p/foo
3. chmod g+s /tmp/p/foo
4. virt-make-fs --floppy /tmp/p /tmp/floppy.img

Actual results:

libguestfs: error: tar_in: tar subcommand failed on directory: /: tar: ./foo: Cannot change mode to rw-rwSr--: Operation not permitted
tar: Exiting with failure status due to previous errors

Expected results:

It should probably ignore the g+s bit

Additional info:

Also happens with the u+s bit:

$ chmod u+s /tmp/p/bar
$ virt-make-fs --floppy /tmp/p /tmp/floppy.img
libguestfs: error: tar_in: tar subcommand failed on directory: /: tar: ./bar: Cannot change mode to rwSrw-r--: Operation not permitted
tar: Exiting with failure status due to previous errors