symbol parsing improvement

This gets rid of a FIXME and K4056 internal bug report.

Allow the comma to be quoted as follows:
     SYMBOLS=S_ice:\,
or
     SYMBOLS=S_ice:','

Disclaimer:
The use of the comma on the map could conflict with future
use of that currently unused symbol for other intended purposes.

src/options.c

@@ -6491,6 +6491,8 @@ escapes(const char *cp, /* might be 'tp', updating in place */
             case 'r':
                 cval = '\r';
                 break;
+            case ',':
+                cval = ',';
             default:
                 cval = *cp;
             }