fix mremap memory synchronization and use of variadic argument

since mremap with the MREMAP_FIXED flag is an operation that unmaps
existing mappings, it needs to use the vm lock mechanism to ensure
that any in-progress synchronization operations using vm identities
from before the call have finished.

also, the variadic argument was erroneously being read even if the
MREMAP_FIXED flag was not passed. in practice this didn't break
anything, but it's UB and in theory LTO could turn it into a hard
error.

prevent allocs than PTRDIFF_MAX via mremap

It's quite feasible for this to happen via MREMAP_MAYMOVE.

global cleanup to use the new syscall interface

initial check-in, version 0.5.0