cmdline: make -F type= accept ;charset=

The -F option allows some custom parameters within the given string, and
those strings are separated with semicolons. You can for example specify
"name=daniel;type=text/plain" to set content-type for the
field. However, the use of semicolons like that made it not work fine if
you specified one within the content-type, like for:
"name=daniel;type=text/plain;charset=UTF-8"
... as the second one would be seen as a separator and "charset" is no
parameter curl knows anything about so it was just silently discarded.

The new logic now checks if the semicolon and following keyword looks
like a parameter it knows about and if it isn't it is assumed to be
meant to be used within the content-type string itself.

I modified test case 186 to verify that this works as intended.

Reported by: Larry Stone
Bug: http://curl.haxx.se/bug/view.cgi?id=3048988

src/main.c

@@ -1202,9 +1202,17 @@ static int formparse(struct Configurable *config,
                 FreeMultiInfo (multi_start);
                 return 2; /* illegal content-type syntax! */
               }
+
               /* now point beyond the content-type specifier */
               sep = (char *)type + strlen(major)+strlen(minor)+1;
 
+              /* there's a semicolon following - we check if it is a filename
+                 specified and if not we simply assume that it is text that
+                 the user wants included in the type and include that too up
+                 to the next zero or semicolon. */
+              if((*sep==';') && !curlx_strnequal(";filname=", sep, 9))
+                sep = strchr(sep+1, ';');
+
               if(*sep) {
                 *sep=0; /* zero terminate type string */