Use a single, consistent approach to returning success versus failure in
vm_map_madvise(). Previously, vm_map_madvise() used a traditional Unix-
style "return (0);" to indicate success in the common case, but Mach-
style return values in the edge cases. Since KERN_SUCCESS equals zero,
the only problem with this inconsistency was stylistic. vm_map_madvise()
has exactly two callers in the entire source tree, and only one of them
cares about the return value. That caller, kern_madvise(), can be
simplified if vm_map_madvise() consistently uses Unix-style return
values.

Since vm_map_madvise() uses the variable modify_map as a Boolean, make it
one.

Eliminate a redundant error check from kern_madvise(). Add a comment
explaining where the check is performed.

Explicitly note that exec_release_args_kva() doesn't care about
vm_map_madvise()'s return value. Since MADV_FREE is passed as the
behavior, the return value will always be zero.

Reviewed by: kib, markj
MFC after: 7 days