Adhere to the naming conventions in uv_timer_* functions.

In the src/*/timer.c code the first argument is called handle, not timer.
We should be consistent in the interface definition file.
Return the errorcode provided by uv_set_artificial_error.

This reduces the line count.
unix, windows: make uv_now() return uint64_t

Using int64_t doesn't make sense here because the return value is never
negative.
unix, windows: change timer intervals to uint64_t
unix, windows: make uv_timer_get_repeat() const correct
doc: document _LARGEFILE_SOURCE + _FILE_OFFSET_BITS
Add common uv_udp_* error checking.