mch_write -> term_write

Switch from POSIX's write() to fwrite(stdout,...) and disable buffering
since vim buffers output explicitly and flushes when needed, like when a
key is typed.
mch_init -> term_init
update mch_suspend

Use the portable uv_kill.
Don't bother with archaic BeOS.
mch_early_init() -> early_init().

Move general initialization functions to early_init, which simplifies
test/unit/helpers.lua, which requires all these functions.