shell: Rewrite client_exec as client_launch

shell: Rewrite client_exec as client_launch

This patch is a major rewrite of the client_exec which fixes a couple of
issues found by upstream.

Specifically this address the following two issues:

- Do not weston_log() after fork()
- Own the session for the launched client

These two issues were integrated into this single patch.

It makes use of previously added wrappers to handle custom environment
being passed to the (shell) client being executed.

(Based on the work from https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/954
and from https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/817)

Bug-AGL: SPEC-4509

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I29d4bcacba3671f67bb915bdb55a80b556e143ac