runtime: simply user throws, expand runtime throws

runtime: simply user throws, expand runtime throws

This gives explicit names to the possible states of throwing (-1, 0, 1).

m.throwing is now one of:

throwTypeOff: not throwing, previously == 0
throwTypeUser: user throw, previously == -1
throwTypeRuntime: runtime throw, previously == 1

For runtime throws, we now always include frame metadata and system
goroutines regardless of GOTRACEBACK to aid in debugging the runtime.

For user throws, we no longer include frame metadata or runtime frames,
unless GOTRACEBACK=system or higher.

For #51485.

Change-Id: If252e2377a0b6385ce7756b937929be4273a56c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/390421
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>