Implement os_microdelay and os_delay on top of it
Remove `RealWaitForChar` and `mch_new_shellsize`

The last occurrence of `RealWaitForChar` was replaced by the `os_microdelay`
function. `mch_new_shellsize` had an empty body, so there seems to be no reason
for keeping it around
Add comments and fix `os_microdelay` call
Remove unneeded variable `in_os_delay`

This variable only existed for the old SIGQUIT handler. Now it's no longer
necessary, especially when Neovim stops being a terminal program.