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.

auth/aix_auth.c

@@ -67,7 +67,7 @@ aixauth_verify(pw, prompt, auth)
     char *message, *pass;
     int reenter = 1;
 
-    pass = tgetpass(prompt, PASSWORD_TIMEOUT * 60, 1);
+    pass = tgetpass(prompt, sudo_inttable[I_PW_TIMEOUT] * 60, 1);
     if (authenticate(pw->pw_name, pass, &reenter, &message) == 0)
 	return(AUTH_SUCCESS);
     else