signal: don't attempt to catch SIGSTOP
signal: don't attempt to catch SIGSTOP

In this commit, we modify the set of default signals we attempt to catch in
order to execute a graceful shutdown. Before this commit, we would attempt
to catch/register for `SIGSTOP`. There're two issues with this

  1. `SIGSTOP` is meant to be used in combination with `SIGCONCT` to allow a
  process to be paused/resumed. Therefore, our action of shutting down once
  received was incorrect.

  2. `SIGSTOP` doesn't exist on windows, so users aren't able to compile on
  this platform without modifying the codebase.

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd