Add command to go home.

When w3m is launched, if no other options are specified, it attempts to
read HTTP_HOME and WWW_HOME from the environment and upon finding a
value for one of these load the url specified.  Once launched, though,
w3m provides no convenience for navigating to the home page.

Here, that ability is added.  A new command GOTO_HOME is defined with a
default key binding of C-_.

proto.h

@@ -81,6 +81,7 @@ extern void prevBf(void);
 extern void backBf(void);
 extern void deletePrevBuf(void);
 extern void goURL(void);
+extern void goHome(void);
 extern void gorURL(void);
 extern void ldBmark(void);
 extern void adBmark(void);