jail.h: New SYSCTL_JAIL_PARAM_DECL()

Like SYSCTL_DECL(), additionally prepending the well-known jail
parameters' sysctl prefix.

Reviewed by: jamie
Approved by: markj (mentor)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46902

(cherry picked from commit f7bda491ef05717fb4fe6c89083485599db951b8)