BUG/MINOR: mworker: fix a FD leak of a sockpair upon a failed reload

BUG/MINOR: mworker: fix a FD leak of a sockpair upon a failed reload

When starting HAProxy in master-worker, the master pre-allocate a struct
mworker_proc and do a socketpair() before the configuration parsing. If
the configuration loading failed, the FD are never closed because they
aren't part of listener, they are not even in the fdtab.

This patch fixes the issue by cleaning the mworker_proc structure that
were not asssigned a process, and closing its FDs.

Must be backported as far as 2.0, the srv_drop() only frees the memory
and could be dropped since it's done before an exec().