Show signal sender for SI_FROMUSER signals.

Suicide doesn't change:

  signal 6 (SIGABRT), code -6 (SI_TKILL), fault addr --------

But homicide now looks like this (this is `sleep 666` killed by
`kill -SEGV` as root:

  signal 11 (SIGSEGV), code 0 (SI_USER from pid 4446, uid 0), fault addr --------

Bug: http://b/78594105
Test: manual
Change-Id: I8c2feafba8cc5a3db85e8250004d428a464c5d9e