Refactor error messages to avoid macros.

Refactor error messages to avoid macros.

* src/m4.h (_): Define, as a placeholder for now.
(M4ERROR, M4ERROR_AT_LINE): Delete.
(m4_error, m4_error_at_line): Change prototype.
(m4_warn, m4_warn_at_line): New prototypes.
* src/m4.c (m4_verror_at_line): New helper function.
(m4_error, m4_error_at_line): Use new function, and properly call
va_end.
(m4_warn, m4_warn_at_line): New functions.
(stackovf_handler, main): All callers changed.
* src/builtin.c: Likewise.
* src/debug.c: Likewise.
* src/eval.c: Likewise.
* src/format.c: Likewise.
* src/freeze.c: Likewise.
* src/input.c: Likewise.
* src/macro.c: Likewise.
* doc/m4.texinfo (Indir, Builtin, Dumpdef, Incr, Eval, Substr)
(Improved forloop): Adjust tests accordingly.

Signed-off-by: Eric Blake <ebb9@byu.net>