tool_help: handle longer lines, exit on too long

Follow-up to 9a0cf56471

- increase the buffer to handle 160 characters manpage lines
- add another assert
- if the line buffer gets full, abort

Ideally, we add another step in the build process that makes the build
fail if this long lines are used.

Closes #14422

src/tool_help.h

@@ -37,7 +37,7 @@ struct scan_ctx {
   size_t elen;
   size_t olen;
   char rbuf[40];
-  char obuf[80];
+  char obuf[160];
   unsigned char show; /* start as at 0.
                          trigger match moves it to 1
                          arg match moves it to 2