MFC r339506, r339508, r340064 (by markj), r340546 (by alc):
Tidy up code to merge vm_entry neighbors and simplify related checks.

r339506:
Reduce code duplication in merging vm_entry neighbors.
r339508:
Unindent.
r340064:
Initialize the eflags field of vm_map headers.
r340546:
Tidy up vm_map_simplify_entry() and its recently introduced helper functions.

Discussed with: markj