eject: use libmount context API for umount

eject: use libmount context API for umount

libmount provides a context API that can be used to replace
the fork+exec code pattern needed to leverage umount(8).
This does not only simplify the code base but also removes
the concern for signal handling due to a wait(2) call, avoids
forking and allocating additional system resources, reduces
security management and makes the code more consistent as other
functions were already using some libmount functionalities.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>