Using reflect.MethodByName disables some linked deadcode optimizations
and makes our binaries much bigger.
Difference before/after this commit:
```
-rwxr-xr-x 1 awly awly 30M Dec 19 15:28 tailscaled.after*
-rwxr-xr-x 1 awly awly 43M Dec 19 15:27 tailscaled.before*
```
Fixes#10627
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>