Allow udprpc to send Jefe.request_reset

[jgallagher]

Prior to this PR, attempting to call Jefe.request_reset via udprpc would time out:

% cargo xtask humility app/gimletlet/app.toml  -- --ip fe80::0c1d:7dff:feef:9f1d%eth0 hiffy -c Jefe.request_reset
    Finished dev [optimized + debuginfo] target(s) in 0.21s
     Running `target/debug/xtask humility app/gimletlet/app.toml -- --ip 'fe80::0c1d:7dff:feef:9f1d%eth0' hiffy -c Jefe.request_reset`
humility: connecting to fe80::0c1d:7dff:feef:9f1d%eth0
humility hiffy failed: Resource temporarily unavailable (os error 11)
Error: humility failed

because jefe returned an access violation:

14 udprpc                       1   6 FAULT: reply fault: task id #0/gen0, reason AccessViolation (was: wait: reply from jefe/gen0)

After this PR, attempting to call Jefe.request_reset via udprpc still times out from humility's point of view, but because the SP has actually reset and therefore does not respond to the request.

[jgallagher]

This isn't the full support requested in #1290 but is better than nothing, I think.