Fix y/n questions being skipped after the first one

Every y/n prompt after the first in a session answered itself with its
default and left the keypress queued for the next command line.

MIN and TIME are not part of the canonical/non-canonical switch: they
live in their own slots of the control-character array and survive
`stty icanon`. drainTTY left them at "min 0 time 0" — return whatever is
buffered, do not wait — and the restore named only icanon and echo, so
the next read returned zero bytes without ever waiting for a key.

Set MIN and TIME explicitly on the way in, and restore the terminal from
the state captured with `stty -g` instead of naming the flags we changed;
mgsh now hands the terminal back exactly as it found it, where before
"min 0" outlived mgsh itself and broke the next program's single-key
reads too.

getkey also reports whether it got a key at all. When it did not, the
answer is no whatever the default says: a question nobody saw must not
be taken as consent. All five call sites default to no, so the bug never
destroyed anything — it only made agreeing impossible.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

version.txt

@@ -1 +1 @@
-4.0.56
+4.0.57