unshare: support the switching of namespaces

In addition to the unshare syscall, there exists the setns syscall, which
allows processes to migrate to the namepsaces of other processes.  Add this
functionality into the unshare command, as they operate in a fairly simmilar
fashion.

Note: There was discussion of adding a path based namespace argument to unshare
in the origional discussion thread, but I opted to leave that out as it didn't
seem to fit in nicely with the current argument pattern.  I figure we can always
add that in later if we need to

[kzak@redhat.com: - fix optional arguments
                  - do not call unshare if no flag specified
                  - use O_CLOEXEC
                  - codding style cleanup]

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: Karel Zak <kzak@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>