errors: support indented context info in m4 macros

errors: support indented context info in m4 macros

* TODO: Address the issue, so remove it.
* data/bison.m4: Use b4_error with [[note]] rather than a complain_at
for context information.
* src/complain.c (complain_args): Take an additional argument, an
indentation pointer, to allow the dispatching of context information.
* src/complain.h (complain_args): Adjust prototype.
* src/scan-skel.l (at_directive_perform): Recognize the new @note mark.
* tests/input.at: Adjust.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>