MAC/do: Factor out setting/destroying rule structures

This generally removes duplicate code and clarifies higher-level
operations, allowing to fix several important bugs.

New (internal) functions:

• ensure_rules(): Ensure that a jail has a populated 'mac_do_osd_jail_slot', and returns the corresponding 'struct rules'.
• dealloc_rules(): Destroy the 'mac_do_osd_jail_slot' slot of a jail.
• set_rules(): Assign already parsed rules to a jail. Leverages ensure_rules().
• parse_and_set_rules(): Combination of parse_rules() and set_rules().

Bugs fixed in mac_do_prison_set():

• A panic if "mdo" is explicitly passed to JAIL_SYS_NEW but "mdo.rules" is absent, in which case 'rules_string' wasn't set (setting 'rules' at this point would do nothing).
• In the JAIL_SYS_NEW case, would release the prison lock and reacquire it, but still using the same 'rules' pointer that can have been freed and changed concurrently, as the prison lock is temporary unlocked. (This is generally a bug of the mac_do_alloc_prison()'s interface when 'lrp' is not NULL.)

Suppress mac_do_alloc_prison(), as it has the following bugs:

• The interface bug mentioned just above.
• Wrong locking, leading to deadlocks in case of setting jail parameters multiple times (concurrently or not).

It has been replaced by either parse_and_set_rules(), or by
ensure_rules() directly coupled with prison_unlock().

Rename mac_do_dealloc_prison(), the OSD destructor, to dealloc_osd(),
and make it free the 'struct rules' itself (which was leaking).

While here, in parse_rules(): Clarify the contract by adding comments,
and check (again) for the rules specification's length.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47597