Implement mprotect for sel4 rumprun

Mprotect is used to implement guard pages by pthreads, and noone likes
running off the stack.

As part of this change the default page size is set to the rump system
page size such that pages can be unmapped and remapped without being broken up and
retyped. Previously the page size was the largest for the amount of
memory given to rump.

Rumprun will still use large pages if CONFIG_USE_LARGE_PAGES is set,
and mprotect is not implemented.

Move mmap emulation into the kernel.

That way systems which want to use kernel-only mode can
use the mmap emulation.

part of deferpanic/gorump#18

sprinkle more debug prints to mmap

remember to initialize too

Make mmap chunks partially unmappable, issue #41

Apparently at least erlang wants this behavior, so we unfortunately
need to emulate it or risk memory corruption. Should adjust the page
allocator to allow non-pow2 free'ing.

make pagesize() a __constfunc

Use page allocator directly for mmap instead of malloc.

We're allocating whole pages anyway, and this way we don't need to pay
for the mmap overhead.

Improve mmap emulation.

+ sanity-check a few more parameters and return errors if insane
+ explicitly allocate full "pages" (malloc did that for us already)
+ zero-fill memory after end of file (per spec)

And now with an attempt to get the msync() symbol name correct.

add msync() "emulation"

split mman syscall emulation into its own module: syscall_mman.c