indent: add helper functions for doing the actual output

This allows to add debug logging to these few functions instead of all
other places that might output something.

Reducing the possible output formats to a few primitives makes dump_line
simpler, especially the fprintf calls.  It also removes the non-constant
printf string.

The call to output_int may be meant for debugging, as the character 0x80
is unlikely to appear in any real-world code.

No functional change.

(rillig)