Remove the VM map zone.

Today, the zone is only used to allocate a trio of kernel maps: the
kernel map itself, and the exec and pipe submaps. Maps for user
processes are dynamically allocated but are embedded in the vmspace
structure, which is allocated from its own zone. Make the
aforementioned kernel maps statically allocated and get rid of the zone.

While here, remove a stale comment above vmspace_alloc() and change the
names of locks initialized in vm_map_init() to match vmspace_zinit().

Reported by: alc
Reviewed by: alc, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26052