tty: Ctrl+Alt+Shift+F1..F6, and VT_SETMODE actually works

Alt+1..6 was a worse choice than the Alt+F-keys it replaced, and the reason
is the one that matters: anything the kernel swallows is taken from every
program that will ever run here, permanently.

Alt+<digit> is Meta-digit. zsh's own line editor binds it as digit-argument --
Alt+1 Alt+2 x repeats x twelve times -- and every readline editor does the
same. Yesterday's chord broke that system-wide, which is exactly the failure
mode of picking a chord applications want.

Working the whole space instead of one axis of it:

  the HOST takes    Alt+F4, Alt+F2, Ctrl+Alt+F1..F6
  APPLICATIONS want Alt+anything (Meta), Ctrl+letter, Ctrl+Shift+digit
                    (Ctrl+Shift+2 types ^@), bare F-keys
  the LAYOUT wants  AltGr+letter -- Polish diacritics

Ctrl+Alt+Shift+F1..F6 is in none of those sets. Display managers bind the same
chord without Shift, so adding it steps out of the host's way, and no terminal
application binds a triple-modified function key.

Which chords actually reach a guest was measured rather than assumed: a
temporary probe in kbdgetc logged every modified keypress and QEMU's sendkey
fired each candidate at it. Everything reached the guest, Ctrl+Alt+F2
included -- so that conflict is the host's display manager, not QEMU. The same
probe corrected an earlier claim: `sendkey altgr-2` produces no event at all,
so the previous commit's "AltGr+2 did not fire" was never actually tested.

And the escape hatch is real now. VT_SETMODE was accepted and ignored, so a
program could ask to own the chord, be told yes, and still not get it.
VT_PROCESS genuinely stops the kernel switching. Together with the EVIOCGRAB
path -- which kbdgetc already checks before the chord is recognised -- there
are three honest layers: nothing, VT_PROCESS, exclusive grab.

What VT_PROCESS still does not do is the VT_RELDISP handshake; struct vt_mode
carries relsig/acqsig for when Schcom needs to release the GPU before the
screen changes hands.

Verified by screendump hash: Alt+2 reaches zsh as Meta, Ctrl+Alt+F2 and
Ctrl+Shift+F2 leave the screen untouched, Ctrl+Alt+Shift+F2 switches.
schfs-check passes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

include/posix.h

@@ -133,6 +133,21 @@ struct rlimit {
 #define VT_ACTIVATE  0x5606
 #define VT_WAITACTIVE 0x5607
 
+// VT_SETMODE modes. VT_AUTO is the kernel switching on its own; VT_PROCESS
+// means a program has taken switching over and the kernel must keep its hands
+// off the keyboard.
+#define VT_AUTO      0x00
+#define VT_PROCESS   0x01
+#define VT_ACKACQ    0x02
+
+struct vt_mode {
+    char  mode;               // VT_AUTO or VT_PROCESS
+    char  waitv;              // unused here
+    short relsig;             // signal on release -- not delivered yet
+    short acqsig;             // signal on acquire -- not delivered yet
+    short frsig;              // unused
+};
+
 struct vt_stat {
     uint16 v_active;          // active terminal, 1-based
     uint16 v_signal;          // unused; Linux keeps it for VT_SETMODE