We never allowed the warnings, only made them not denied.

…mertj

libstd fuchsia fixes

This fixes two bugs in libstd on Fuchsia:

- `zx_time_t` was changed to an `i64`, but this never made it into libstd
- When spawning processes where any of the stdio were null, libstd attempts to open `/dev/null`, which doesn't exist on Fuchsia

r? @cramertj

…crichton

Ensure edition lints and internal lints are enabled with deny-warnings=false

Previously we only passed the deny command line flags if deny-warnings was enabled, but now we either pass -W... or -D... for each of the flags as appropriate.

This is also a breaking change to x.py as it changes `--warnings=allow` to `--warnings=warn` which is what that flag actually did; we don't have an allow warnings mode.

Update rustc-guide

r? @ehuss

Upgrade env_logger to 0.6