The opt_*.h headers must be included before any system header, except
sys/cdefs.h.  In particular, in case of COMPAT_43, param.h includes
sys/types.h, which includes sys/select.h, which includes
sys/_sigset.h.  The _sigset.h customizes the provided definions based
on COMPAT_43, eliminating osigset_t if symbol is not defined.  The
sys/proc.h is included after opt_compat.h and needs osigset_t.

Move opt_compat.h inclusion into the right place.

Sponsored by:	The FreeBSD Foundation