warnings: don't complain about m4_foo and b4_foo when from the user
Currently, occurrences of these identifiers in the user's input yield
spurious warnings.
To tell the difference between a legitimate m4_foo from the user, and
a bad m4_foo coming from a non-evaluated macro of a skeleton, escape
the user's identifiers as m4@'_foo. We already use @' as a special
sequence to be stripped from the skeleton's output.
See <https://lists.gnu.org/r/bug-bison/2021-10/msg00026.html> and
previous commit ("warnings: be less picky about occurrences of m4_/b4_
in the output").
* src/flex-scanner.h (OBSTACK_SGROW): New.
* src/output.c (output_escaped): Escape m4_ and b4_.
* src/scan-code.l: Likewise.
* src/system.h (obstack_escape): Likewise.
And rewrite as a function.
* tests/skeletons.at (Suspicious sequences): Make sure the user can
use m4_foo/b4_foo without spurious warnings.