MEDIUM: errors: get rid of shm_open()

MEDIUM: errors: get rid of shm_open()

Since 5ee266b7 ("MINOR: error: simplify startup_logs_init_shm"), the FD
of the startup logs is always closed and the HAPROXY_STARTUPLOGS_FD
variable is not used anymore. Which means we only need a mmap.

Indeed the shm_open() function was only needed to keep the shm between
the exec() of the master so we can get the logs stored there after doing
the final exec() in wait mode. Since the wait mode doesn't exist
anymore and the parsing is done in a worker, we only need to share a
memory zone between the master and the worker.

This patch removes shm_open() and replace it with a simple mmap(), this
way the shared startup-logs become more portable and USE_SHM_OPEN is not
required anymore.