unix, windows: return UV_EINVAL, not UV_EFAULT

A wrong multicast membership should return EINVAL.
build: don't export _POSIX_C_SOURCE on non-linux

Don't export _POSIX_C_SOURCE to dependents. On the BSDs (and the
BSD-ish, like OS X), it hides SysV and BSD definitions.

The exception is Linux: _POSIX_C_SOURCE=200112 unlocks some of the
newer pthreads features like spinlocks and barriers, so keep
exporting it on that platform.
test: remove .travis.yml

Travis CI was generating a lot of false positives (or rather, false
negatives - tests that fail due to the environment they run in) and
it's been in a continuous error state for days now.

Remove it, we'll set up something ourselves.
unix: support sending uv_udp_t handles over ipc pipes
unix: fix style issues in udp.c