BUG/MINOR: mworker: PROC_O_LEAVING used but not updated

BUG/MINOR: mworker: PROC_O_LEAVING used but not updated

Since commit 2be557f ("MEDIUM: mworker: seamless reload use the internal
sockpair"), we are using the PROC_O_LEAVING flag to determine which
sockpair worker will be used with -x during the next reload.

However in mworker_reexec(), the PROC_O_LEAVING flag is not updated, it
is only updated at startup in mworker_env_to_proc_list().

This could be a problem when a remaining process is still in the list,
it could be selected as the current worker, and its socket will be used
even if _getsocks doesn't work anymore on it. (bug #1803)

This patch fixes the issue by updating the PROC_O_LEAVING flag in
mworker_proc_list_to_env() just before using it in mworker_reexec()

Must be backported to 2.6.