uwrap: Disable uwrap only for thread which calls fork().

When fork() is called here is no need to disable uwrap as a whole.
This change disables only uwrap for thread which called fork().

TESTS: Add new test test_pthread_atfork.
This test aims to multithreaded applications which  creates threads
and then need call fork() and execve() functions.

Signed-off-by: Robin Hack <hack.robin@gmail.com>