child_process: improve killSignal validations

As it is, the `killSignal` is just retrieved from an object and used.
If the signal passed is actually one of the inherited properties of
that object, Node.js will die. For example,

    ➜  node -e "child_process.spawnSync('ls', {killSignal: 'toString'})"
    Assertion failed: (0), function uv_close, file ....core.c, line 166.
    [1]    58938 abort      node -e "child_process.spawnSync(...)"

1. This patch makes sure that the signal is actually a own property of
   the constants object.

2. Extends the killSignal validation to all the other functions.

PR-URL: https://github.com/nodejs/node/pull/10423



Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>