Introduce a new sysctl to retrieve VM map entries

This adds a new sysctl KERN_PROC_VMMAP, which returns an array of VM map
entries of a specified process.  This prevents debuggers from iterating
vm_map_entry RB tree via kvm(3).

The name KERN_PROC_VMMAP and struct kinfo_vmentry are chosen from the same
function in FreeBSD.  struct kinfo_vmentry is revised to reduce size, because
OpenBSD does not keep track of filepaths.  The semantic is also changed to
return max buffer size as a hint, and start iteration at the specified base
address.

Much valuable input from deraadt@, guenther@, tedu@

OK tedu@ deraadt@