diagnostics: check the styling

Enable checking of styles even when libtextstyle is not installed.

* src/getargs.h, src/getargs.c (style_debug): New.
(getargs_colors): Set it when --style=debug.
* src/complain.c (begin_use_class, end_use_class): Use it.
* tests/diagnostics.at: New.

tests/local.mk

@@ -48,6 +48,7 @@ TESTSUITE_AT =                                \
   %D%/calc.at                                 \
   %D%/conflicts.at                            \
   %D%/cxx-type.at                             \
+  %D%/diagnostics.at                          \
   %D%/existing.at                             \
   %D%/glr-regression.at                       \
   %D%/headers.at                              \