support colons in SGR character attributes

Patch by Mikhail Kot <to@myrrc.dev>
With some modifications to behave more like xterm (see note below).

Example:

	printf '\033[48;2;255:0:0mtest\n'

https://invisible-island.net/xterm/ctlseqs/ctlseqs.html

Some notes:

"CSI Pm m  Character Attributes (SGR).
[...]
o   xterm allows either colons (standard) or semicolons
(legacy) to separate the subparameters (but after the
first colon, colons must be used).

st.c

@@ -1132,6 +1132,7 @@ csiparse(void)
 {
 	char *p = csiescseq.buf, *np;
 	long int v;
+	int sep = ';'; /* colon or semi-colon, but not both */
 
 	csiescseq.narg = 0;
 	if (*p == '?') {
@@ -1149,7 +1150,9 @@ csiparse(void)
 			v = -1;
 		csiescseq.arg[csiescseq.narg++] = v;
 		p = np;
-		if (*p != ';' || csiescseq.narg == ESC_ARG_SIZ)
+		if (sep == ';' && *p == ':')
+			sep = ':'; /* allow override to colon once */
+		if (*p != sep || csiescseq.narg == ESC_ARG_SIZ)
 			break;
 		p++;
 	}