Add some documentetion to tsetmode

The names of the terminal modes supported by vt102 are (taken from the VT220
programmer reference manual):

Table 4-7 ANSI-Standardized Modes
Name  Mnemonic		    Parameter (Ps)
Error (ignored)		    -         0 (3/0)
Keyboard action		    KAM	      2 (3/2)
Insert/replace		    IRM	      4 (3/4)
Send/receive		    SRM	      12 (3/1 3/2)
Line feed/new line	    LNM       20 (3/2 3/0)

Table 4-8 ANSI-Compatible DEC Private Modes
Name  Mnemonic		  Parameter (Ps)
Error (ignored)		  -	    0 (3/0)
Cursor key		  DECCKM    1 (3/1)
ANSI/VT52		  DECANM    2 (3/2)
Column			  DECCOLM   3 (3/3)
Scroll			  DECSCLM   4 (3/4)
Screen			  DECSCNM   5 (3/5)
Origin			  DECOM	    6 (3/6)
Auto wrap		  DECAWM    7 (3/7)
Auto repeat		  DECARM    8 (3/8)
Printer form feed	  DECPFF    18 (3/1 3/8)
Printer extent		  DECPEX    19 (3/1 3/9)
Text cursor enable	  DECTCEM   25 (3/2 3/5)
National replacement character sets DECNRCM 42 (3/4 3/2)

This patch adds a comment for each one of these sequences.
---
 st.c |   26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

st.c

@@ -1280,7 +1280,8 @@ tsetmode(bool priv, bool set, int *args, int narg) {
 	for(lim = args + narg; args < lim; ++args) {
 		if(priv) {
 			switch(*args) {
-			case 1:
+				break;
+			case 1: /* DECCKM -- Cursor key */
 				MODBIT(term.mode, set, MODE_APPKEYPAD);
 				break;
 			case 5: /* DECSCNM -- Reverve video */
@@ -1289,9 +1290,14 @@ tsetmode(bool priv, bool set, int *args, int narg) {
 				if(mode != term.mode)
 					redraw();
 				break;
-			case 7:
+			case 6: /* XXX: DECOM -- Origin */
+				break;
+			case 7: /* DECAWM -- Auto wrap */
 				MODBIT(term.mode, set, MODE_WRAP);
 				break;
+			case 8: /* XXX: DECARM -- Auto repeat */
+				break;
+			case 0:  /* Error (IGNORED) */
 			case 12: /* att610 -- Start blinking cursor (IGNORED) */
 				break;
 			case 25:
@@ -1319,6 +1325,12 @@ tsetmode(bool priv, bool set, int *args, int narg) {
 				tcursor((set) ? CURSOR_SAVE : CURSOR_LOAD);
 				break;
 			default:
+			/* case 2:  DECANM -- ANSI/VT52 (NOT SUPPOURTED) */
+			/* case 3:  DECCOLM -- Column  (NOT SUPPORTED) */
+			/* case 4:  DECSCLM -- Scroll (NOT SUPPORTED) */
+			/* case 18: DECPFF -- Printer feed (NOT SUPPORTED) */
+			/* case 19: DECPEX -- Printer extent (NOT SUPPORTED) */
+			/* case 42: DECNRCM -- National characters (NOT SUPPORTED) */
 				fprintf(stderr,
 					"erresc: unknown private set/reset mode %d\n",
 					*args);
@@ -1326,13 +1338,17 @@ tsetmode(bool priv, bool set, int *args, int narg) {
 			}
 		} else {
 			switch(*args) {
-			case 2:
+			case 0:  /* Error (IGNORED) */
+				break;
+			case 2:  /* KAM -- keyboard action */
 				MODBIT(term.mode, set, MODE_KBDLOCK);
 				break;
-			case 4:
+			case 4:  /* IRM -- Insertion-replacement */
 				MODBIT(term.mode, set, MODE_INSERT);
 				break;
-			case 20:
+			case 12: /* XXX: SRM -- Send/Receive */
+				break;
+			case 20: /* LNM -- Linefeed/new line */
 				MODBIT(term.mode, set, MODE_CRLF);
 				break;
 			default: