template: use new format for boolean, optional and subtokens

Use the opt-token format for boolean tokens (replacing %opt-* and
%nopt-*)
template: add signature addition

-add a signature addition
-change formatting to raggedright
-include letter.lco
-set sigindent in letter.lco
server: add debugging flag

if debugging is enabled, letter.php won't generate a PDF file but will
print the compiler's output. Also, temporary files won't be deleted.