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