Add support for "Defaults" line in sudoers to make configuration variables

changable at runtime (and on a global, per-host and per-user basis).
Both the names and the internal representation are still subject to change.
It was necessary to make sudo_user.runas but a char ** instead of a
char * since this value can be changed by a Defaults line.  There is a
similar (but more complicated) issue with sudo_user.prompt but it
is handled differently at the moment.

Add a "-L" flag to list the name of options with their descriptions.  This
may only be temporary.

Move some prototypes to parse.h

Be much less restrictive on what is allowed for a username.

parse.h

@@ -104,4 +104,12 @@ struct generic_alias {
 extern struct matchstack *match;
 extern int top;
 
+/*
+ * Prototypes
+ */
+int addr_matches	__P((char *));
+int command_matches	__P((char *, char *, char *, char *));
+int netgr_matches	__P((char *, char *, char *));
+int usergr_matches	__P((char *, char *));
+
 #endif /* _SUDO_PARSE_H */