Replace /urlopen with /url and adapt /executable

/urlopen is replaced by /url with the following sub commands:
/url open <url>
/url save <url> [<path>]

Signed-off-by: Pierre Mazière <pierre.maziere@gmx.com>

src/config/preferences.h

@@ -172,6 +172,7 @@ typedef enum {
     PREF_SLASH_GUARD,
     PREF_MAM,
     PREF_URL_OPEN_CMD,
+    PREF_URL_SAVE_CMD,
 } preference_t;
 
 typedef struct prof_alias_t {