process.spawn: validate unshare calls (bug 673900)

In order to prevent failed unshare calls from corrupting the state
of an essential process, validate the relevant unshare call in a
short-lived subprocess. An unshare call is considered valid if it
successfully executes in a short-lived subprocess.

Bug: https://bugs.gentoo.org/673900
Signed-off-by: Zac Medico <zmedico@gentoo.org>
1 file changed