module: eliminate double `getenv()`

`process.env` access results in a synchronous `getenv` call. Cache the
first result instead and save one syscall.
npm: Upgrade to v1.4.21
v8: Interrupts must not mask stack overflow.

Backport of https://codereview.chromium.org/339883002
uv: Upgrade to v0.10.28
Merge remote-tracking branch 'upstream/v0.10' into v0.10