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.

tests/skeletons.at

@@ -332,14 +332,18 @@ AT_CLEANUP
 
 AT_SETUP([[Suspicious sequences]])
 
+# <https://lists.gnu.org/r/bug-bison/2021-10/msg00026.html>.
+
+# We must escape m4_foo as m4@&t@_foo to avoid Autotest's protection
+# against its own suspicious sequences.
 AT_DATA([[skel.c]],
 [[m4@&t@_include(b4_skeletonsdir/[c.m4])
 m4@&t@_divert_push(0)d@&t@nl
 @output(b4_parser_file_name@)d@&t@nl
 ]b4_user_pre_prologue[
 ]b4_user_post_prologue[
-b4_unevaluated
-m4@&t@_unevaluated
+b4_poison
+m4@&t@_poison
 ]b4_epilogue[
 
 m4@&t@_divert_pop(0)
@@ -348,19 +352,21 @@ m4@&t@_divert_pop(0)
 AT_DATA([[input1.y]],
 [[%skeleton "./skel.c"
 %{
-  myb4_unevaluated
-  mym4_unevaluated
+  myb4_user
+  mym4_user
+  b4_user
+  m4@&t@_user
 %}
 %%
-start: ;
+b4_user: "b4_user";
 %%
-myb4_unevaluated
-mym4_unevaluated
+b4_user
+m4@&t@_user
 ]])
 
 AT_BISON_CHECK([[input1.y]], [], [],
-[[input1.tab.c:10: warning: suspicious sequence in the output: b4_unevaluated [-Wother]
-input1.tab.c:11: warning: suspicious sequence in the output: m4@&t@_unevaluated [-Wother]
+[[input1.tab.c:12: warning: suspicious sequence in the output: b4_poison [-Wother]
+input1.tab.c:13: warning: suspicious sequence in the output: m4@&t@_poison [-Wother]
 ]])