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/sia.c

@@ -80,8 +80,8 @@ sudo_collect(timeout, rendition, title, nprompts, prompts)
     switch (rendition) {
 	case SIAFORM:
 	case SIAONELINER:
-	    if (timeout <= 0 || timeout > PASSWORD_TIMEOUT * 60)
-		timeout = PASSWORD_TIMEOUT * 60;
+	    if (timeout <= 0 || timeout > sudo_inttable[I_PW_TIMEOUT] * 60)
+		timeout = sudo_inttable[I_PW_TIMEOUT] * 60;
 	    /*
 	     * Substitute custom prompt if a) the sudo prompt is not "Password:"
 	     * and b) the SIA prompt is "Password:" (so we know it is safe).