Add a parameter to specify an umask for the child.

Add an `umask` parameter to set the process umask in the forked child,
just before to exec. Defults to `None` to let the umask untouched.

This is useful when spawning processes which create files (e.g.
`genisoimage`), so them can have the correct permissions right
from the start.

Signed-off-by: Francesco Romani <fromani@redhat.com>