Fix: concurrent exec(2) file descriptor leak

Fix: concurrent exec(2) file descriptor leak

If exec(2) is executed by the application concurrently with LTTng-UST
listener threads between the creation of a file descriptor with
socket(2), recvmsg(2), or pipe(2) and call to fcntl(3) FD_CLOEXEC, those
file descriptors will stay open after the exec, which is not intended.

As a consequence, shared memory files for ring buffers can stay present
on the file system for long-running traced processes.

Use:

- pipe2(2) O_CLOEXEC (supported since Linux 2.6.27, and by FreeBSD),
- socket(2) SOCK_CLOEXEC (supported since Linux 2.6.27, and by FreeBSD),
- recvmsg(2) MSG_CMSG_CLOEXEC (supported since Linux 2.6.23 and by FreeBSD),

rather than fcntl(2) FD_CLOEXEC to make sure the file descriptors are
closed on exec immediately upon their creation.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Id2167cf99d7cb8a8425fc0dc13745f023a504562