MFC r190886:
When vm_map_wire(9) is allowed to skip holes in the wired region, skip
the mappings without any of read and execution rights, in particular,
the PROT_NONE entries. This makes mlockall(2) work for the process
address space that has such mappings.

Since protection mode of the entry may change between setting
MAP_ENTRY_IN_TRANSITION and final pass over the region that records
the wire status of the entries, allocate new map entry flag
MAP_ENTRY_WIRE_SKIPPED to mark the skipped PROT_NONE entries.