MINOR: error: simplify startup_logs_init_shm

MINOR: error: simplify startup_logs_init_shm

This patch simplifies the code of startup_logs_init_shm(). We no longer re-exec
master process twice after each reload to free its unused memory, which it had
to allocate, because it has parsed all configuration sections. So, there is no
longer need to keep SHM fd opened between the first and the next reloads. We
can completely remove HAPROXY_STARTUPLOGS_FD.

In step_init_1() we continue to call startup_logs_init_shm() to open SHM and to
allocate startup logs ring area within it. In master-worker mode, worker
duplicates initial startup logs ring after sending its READY state to master.
Sharing the same ring between two processes until the worker finishes its
initialization allows to show at master CLI output worker's startup logs.

During the next reload master process should free the memory allocated for the
ring structure. Then after the execvp() it will reopen and map SHM area again
and it will reallocate again the ring structure.