We used to include panic calls in tracebacks; however, when
runtime.panic was renamed to runtime.gopanic in the conversion of the
runtime to Go, we missed the special case in showframe that includes
panic calls even though they're in package runtime.

Fix the function name check in showframe (and, while we're here, fix
the other check for "runtime.panic" in runtime/pprof). Since the
"runtime.gopanic" name doesn't match what users call panic and hence
isn't very user-friendly, make traceback rewrite it to just "panic".

Updates #5832, #13857. Fixes #14315.

Change-Id: I8059621b41ec043e63d5cfb4cbee479f47f64973
Reviewed-on: https://go-review.googlesource.com/19492
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>