SCP: Add quoted string argument to SET ENV otherwise trim whitespace

Previously, unexpected values might end up being set when using SET ENV
if there were trailing whitespace on the line being parsed.  Any such white
space is now explicitly trimmed before the environment variable is set.
Once we do that, we need to provide a way to deliberately set an environment
variable with trailing spaces.  This is now achieved by using a quoted string
value specified as:

    SET ENV -S var="value  "

The quotes are removed prior to setting the environment variable value.
The contents of the quoted string are parsed the same as EXPECT and SEND
arguments.

scp.c

@@ -4575,8 +4575,20 @@ if (sim_switches & SWMASK ('P')) {
     else
         cptr = deflt;
     }
-else
+else {
     cptr = get_glyph (cptr, varname, '=');              /* get environment variable name */
+    strlcpy (cbuf, cptr, sizeof(cbuf));
+    sim_trim_endspc (cbuf);
+    cptr = cbuf;
+    if (sim_switches & SWMASK ('S')) {                  /* Quote String argument? */
+        uint32 str_size;
+
+        get_glyph_quoted (cptr, cbuf, 0);
+        if (SCPE_OK != sim_decode_quoted_string (cbuf, (uint8 *)cbuf, &str_size)) 
+            return sim_messagef (SCPE_ARG, "Invalid quoted string: %s\n", cbuf);
+        cbuf[str_size] = '\0';
+        }
+    }
 setenv(varname, cptr, 1);
 return SCPE_OK;
 }