Fix long-standing issue with incorrect radix mask calculation.

Usual symptoms are messages like
rn_delete: inconsistent annotation
rn_addmask: mask impossibly already in tree
or inability to flush/delete particular prefix in ipfw table.

Changes:
* Assume 32 bytes as maximum radix key length
* Remove rn_init()
* Statically allocate rn_ones/rn_zeroes
* Make separate mask tree for each "normal" tree instead of system global one
* Remove "optimization" on masks reusage and key zeroying
* Change rn_addmask() arguments to accept tree pointer (no users in base)

PR:		kern/182851, kern/169206, kern/135476, kern/134531
Found by:	Slawa Olhovchenkov <slw@zxy.spb.ru>
MFC after:	2 weeks
Reviewed by:	glebius
Sponsored by:	Yandex LLC