radix_trie: have vm_radix use pctrie code

Implement everything currently in vm_radix.c with calls to functions
in subr_pctrie.c, asccessed via the interface provided by the
DEFINE_PCTRIE_SMR macro.

Add back some #includes removed in the first attempt, and avoid the
use of a discontinued type in a bit of conditionally compiled code.

Reviewed by: alc, markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D41344