unshare: support the time namespace

unshare: support the time namespace

This adds support to unshare for time namespaces. With the newly added
options '-t, --time' and '--monotonic' and '--boottime' it is now
possible to change CLOCK_MONOTONIC and CLOCK_BOOTTIME in a new time
namespace.

The time namespace has been merged in kernel version 5.6 and an easy way
to test it is using CLOCK_BOOTTIME and the uptime command:

 # uptime
 11:08:26 up 20:28,  1 user,  load average: 0.00, 0.00, 0.00
 # ./unshare --fork --time --boottime 100000000 uptime
 11:08:29 up 1158 days,  6:15,  1 user,  load average: 0.00, 0.00, 0.00

Signed-off-by: Adrian Reber <areber@redhat.com>