unshare: use '-T' for time namespace instead of '-t'

unshare: use '-T' for time namespace instead of '-t'

While working on getting time namespace support into 'nsenter' it was
not possible to use '-t' to enter a time namespace as '-t' is the short
option for '--target'. Fortunately '-T' is still available in 'nsenter'
and 'unshare' and therefore let's change 'unshare' to use the same flag
for the time namespace as 'nsenter'.

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