Remove stack option. We no longer use the stack, since the stack was

Remove stack option.  We no longer use the stack, since the stack was
never deeper than 1; instead, use the new auto var c_context to record
the stacked value.

Remove nounput option.  At an unexpected end of file, we now unput
the minimal input necessary to end cleanly; this simplifies the
code.

Avoid unbounded token sizes where this is easy.

(unexpected_end_of_file): New function.
Use it to systematize the error message on unexpected EOF.
(last-string): Now auto, not static.
(YY_OBS_FREE): Remove unnecessary do while (0) wrapper.
(scanner_last_string_free): Remove; not used.
(percent_percent_count): Move decl to just before use.
(SC_ESCAPED_CHARACTER): Return ID at unexpected end of file,
not the (never otherwised-used) CHARACTER.