Fix: Handle EINTR of waitpid when spawning a session daemon

waitpid may fail for various reasons, being interrupted being
the most frequent. In such a case, status is left uninitialized
which results in the WIFSIGNALED and WIFEXITED macros returning
undefined value, resulting in surprising logging statements such
as "killed by signal 114".

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>