unix, windows: make buf arg to uv_fs_write const

Change the uv_fs_write() prototype so the 'buf' argument is now
`const void*` rather than `void*`.

The argument is stored in a non-const field in the uv_fs_t but that's
inconsequential because the memory it points to is not touched.
sunos: fix build breakage introduced in e3a657c
aix: fix build breakage introduced in 3ee4d3f

Possibly, anyway - this change is untested. Without it, libuv
definitely won't build on AIX though.