process: pid can be a string in process.kill()

Not allowing string was a change from v0.10 behaviour, commented on in
joyent/node#7991. Allow them again, but still check that argument is
numberish. Also, simplify the fragile and non-portable test code
introduced in 832ec1cd507 that required fixups 2a415358ee, and
ef3c4ed3d.

Sorry about the test awfulness I introduced, @misterdjules @tjfontaine.
You can send signals to negative pids (see man 2 kill), and negative numbers are tested for in the test, and supported by node/libuv.