api: nvim_get_mode()

Asynchronous API functions are served immediately, which means pending
input could change the state of Nvim shortly after an async API function
result is returned.

nvim_get_mode() is different:
  - If RPCs are known to be blocked, it responds immediately (without
    flushing the input/event queue)
  - else it is handled just-in-time before waiting for input, after
    pending input was processed. This makes the result more reliable
    (but not perfect).

Internally this is handled as a special case, but _semantically_ nothing
has changed: API users never know when input flushes, so this internal
special-case doesn't violate that. As far as API users are concerned,
nvim_get_mode() is just another asynchronous API function.

In all cases nvim_get_mode() never blocks for more than the time it
takes to flush the input/event queue (~µs).

Note: This doesn't address #6166; nvim_get_mode() will provoke #6166 if
e.g. `d` is operator-pending.

Closes #6159

src/nvim/memory.c

@@ -345,10 +345,6 @@ char *xstpcpy(char *restrict dst, const char *restrict src)
 /// WARNING: xstpncpy will ALWAYS write maxlen bytes. If src is shorter than
 /// maxlen, zeroes will be written to the remaining bytes.
 ///
-/// TODO(aktau): I don't see a good reason to have this last behaviour, and
-/// it is potentially wasteful. Could we perhaps deviate from the standard
-/// and not zero the rest of the buffer?
-///
 /// @param dst
 /// @param src
 /// @param maxlen