debug_fuzzer change

I don't always want to abort() on an impossible() when debug_fuzzing,
especially if the first impossible() encountered isn't related to the
bug I'm in the midst of trying to hunt down.

I often have breakpoints on impossible() anyway, and I'd like a simple
way to avoid the panic() call during a lengthy debug session.

Make iflags.debug_fuzzer an xint8 instead of a boolean.
Call abort() only if iflags.debug_fuzzer is set to 1.

That allows setting iflags.debug_fuzzer to 2 in order to bypass the
abort call, and make use of other breakpoints that have been set
to narrow down a particular issue.

src/pline.c

@@ -587,7 +587,8 @@ impossible(const char *s, ...)
     va_end(the_args);
     pbuf[BUFSZ - 1] = '\0'; /* sanity */
     paniclog("impossible", pbuf);
-    if (iflags.debug_fuzzer)
+    if (iflags.debug_fuzzer
+        && iflags.debug_fuzzer == 1)
         panic("%s", pbuf);
 
     gp.pline_flags = URGENT_MESSAGE;