Sync description of PROT_* flags between mmap.2 and mprotect.2

ok guenther
Various small typographic fixes for mman.h manual pages:

Use .Fn instead of .Nm as appropriate
Use .In for include lines
Use .Rv -std where possible
Use .Xr to refer to functions from other manual pages
Remove extraneous sys/types.h include

More substantive changes to follow.

Discussed with schwarze