Support terminals that don't allow disabling attributes.

Both Telemate v4.20 and Qmodem v4.6 don't support these, and Synchronet
supports both of those.

Instead of using the 22, 24, 25, 27, or 28 SGR parameters, always
use the 0 parameter to turn HIGH, UNDERLINE, BLINK, REVERSED, or
CONCEALED (respectively) off.

Fixes issues #934 and #935

src/sbbs3/ansi_terminal.cpp

@@ -107,6 +107,7 @@ const char *ANSI_Terminal::attrstr(unsigned atr)
 	return "-Invalid use of ansi()-";
 }
 
+#if 0 // Used by disabled telemate/qmodem code below
 static uint32_t
 popcnt(const uint32_t val)
 {
@@ -119,6 +120,7 @@ popcnt(const uint32_t val)
 	i *= 0x01010101;
 	return  i >> 24;
 }
+#endif
 
 // Was ansi() and ansi_attr()
 char* ANSI_Terminal::attrstr(unsigned atr, unsigned curatr, char* str, size_t strsz)
@@ -165,8 +167,10 @@ char* ANSI_Terminal::attrstr(unsigned atr, unsigned curatr, char* str, size_t st
 	}
 
 	lastret = strlcpy(str, "\033[", strsz);
+
+// Not supported by Telemate or Qmodem, which we do support
+#if 0
 	uint32_t changed_mask = (curatr ^ atr) & (HIGH | BLINK | REVERSED | UNDERLINE | CONCEALED);
-	// TODO: CSI 0 m does *NOT* set 
 	if (changed_mask) {
 		uint32_t set = popcnt(changed_mask & atr);
 		uint32_t clear = popcnt(changed_mask & ~atr);
@@ -187,10 +191,24 @@ char* ANSI_Terminal::attrstr(unsigned atr, unsigned curatr, char* str, size_t st
 
 		if (set_only_weight < set_clear_weight) {
 			lastret = strlcat(str, "0;", strsz);
-			curatr &= ~0x77;
+			curatr &= ~(0x77 | HIGH | BLINK | REVERSED | UNDERLINE | CONCEALED);
 			curatr |= ANSI_NORMAL;
 		}
 	}
+#else
+	// TODO: Detect or configure this.
+	// If turning off any of the special bits, reset
+	// to normal rather than use the turn off code.
+	if (((!(atr & HIGH)) && (curatr & HIGH)) ||
+	    ((!(atr & BLINK)) && (curatr & BLINK)) ||
+	    ((!(atr & REVERSED)) && (curatr & REVERSED)) ||
+	    ((!(atr & UNDERLINE)) && (curatr & UNDERLINE)) ||
+	    ((!(atr & CONCEALED)) && (curatr & CONCEALED))) {
+		lastret = strlcat(str, "0;", strsz);
+		curatr &= ~(0x77 | HIGH | BLINK | REVERSED | UNDERLINE | CONCEALED);
+		curatr |= ANSI_NORMAL;
+	}
+#endif
 	if (atr & HIGH) {                     /* special attributes */
 		if (!(curatr & HIGH))
 			lastret = strlcat(str, "1;", strsz);