lib: Add tst_umount()

Various desktop deamons (gvfsd-trash is known for that) may be
stupid enough to probe all newly mounted filesystem which results in
umount(2) failing with EBUSY.

Introduce tst_umount() function that behaves exactly as umount(2) but
retries several times on a failure + add documentation.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
3 files changed