Add pctrie_init() and vm_radix_init() to initialize generic pctrie and
vm_radix trie.

Existing vm_radix_init() function is renamed to vm_radix_zinit().
Inlines moved out of the _ headers.

Reviewed by: alc, markj (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D11661