Revamp extended keys support to more closely match xterm and support

mode 2 as well as mode 1. From Stanislav Kljuhhin (GitHub issue 4038).

This changes tmux to always request mode 2 from parent terminal, change
to an unambiguous internal representation of keys, and adds an option
(extended-keys-format) to control the format similar to the xterm(1)
formatOtherKeys resource.

tmux.h

@@ -138,8 +138,7 @@ struct winlink;
 #define KEYC_IMPLIED_META    0x08000000000000ULL
 #define KEYC_BUILD_MODIFIERS 0x10000000000000ULL
 #define KEYC_VI		     0x20000000000000ULL
-#define KEYC_EXTENDED	     0x40000000000000ULL
-#define KEYC_SENT	     0x80000000000000ULL
+#define KEYC_SENT	     0x40000000000000ULL
 
 /* Masks for key bits. */
 #define KEYC_MASK_MODIFIERS  0x00f00000000000ULL
@@ -187,6 +186,42 @@ struct winlink;
  */
 typedef unsigned long long key_code;
 
+/* C0 control characters */
+enum {
+	C0_NUL,
+	C0_SOH,
+	C0_STX,
+	C0_ETX,
+	C0_EOT,
+	C0_ENQ,
+	C0_ASC,
+	C0_BEL,
+	C0_BS,
+	C0_HT,
+	C0_LF,
+	C0_VT,
+	C0_FF,
+	C0_CR,
+	C0_SO,
+	C0_SI,
+	C0_DLE,
+	C0_DC1,
+	C0_DC2,
+	C0_DC3,
+	C0_DC4,
+	C0_NAK,
+	C0_SYN,
+	C0_ETB,
+	C0_CAN,
+	C0_EM,
+	C0_SUB,
+	C0_ESC,
+	C0_FS,
+	C0_GS,
+	C0_RS,
+	C0_US
+};
+
 /* Special key codes. */
 enum {
 	/* Focus events. */
@@ -582,14 +617,16 @@ enum tty_code_code {
 #define MODE_MOUSE_ALL 0x1000
 #define MODE_ORIGIN 0x2000
 #define MODE_CRLF 0x4000
-#define MODE_KEXTENDED 0x8000
+#define MODE_KEYS_EXTENDED 0x8000
 #define MODE_CURSOR_VERY_VISIBLE 0x10000
 #define MODE_CURSOR_BLINKING_SET 0x20000
+#define MODE_KEYS_EXTENDED_2 0x40000
 
 #define ALL_MODES 0xffffff
 #define ALL_MOUSE_MODES (MODE_MOUSE_STANDARD|MODE_MOUSE_BUTTON|MODE_MOUSE_ALL)
 #define MOTION_MOUSE_MODES (MODE_MOUSE_BUTTON|MODE_MOUSE_ALL)
 #define CURSOR_MODES (MODE_CURSOR|MODE_CURSOR_BLINKING|MODE_CURSOR_VERY_VISIBLE)
+#define EXTENDED_KEY_MODES (MODE_KEYS_EXTENDED|MODE_KEYS_EXTENDED_2)
 
 /* Mouse protocol constants. */
 #define MOUSE_PARAM_MAX 0xff