Restoring a multi-threaded process with CRIU's SELinux support fails
because SELinux does not always support changing the process context of
a multi-threaded process.

Reading the man-page for setcon(), to change the context of a running
process, it states that changing the SELinux context of a multi-threaded
process can only work 'if the new security context is bounded by the old
security context'.

To be able to restore a process without the need to have 'the new
security context [] bounded by the old security context', this sets the
SELinux process context before creating the threads. Thus all threads
are created with the process context of the main process.
Signed-off-by: Adrian Reber <areber@redhat.com>