fix various issues in curl bindings related to tile cancelation
use IPv4 address instead of localhost to work around systems that don't resolve localhost to 127.0.0.1
fix undefined value
[skip ci]
[linux] let child process exit when parent hung up
clean up poll handle data in close() function so that assert() won't fail
Merge pull request #544 from mapbox/fix-curl

Update CURL bindings to be more stable