Remove support for "service threads"

Summary: Apparently hphp used to support running background threads written in
PHP, but we don't use this anymore and it might not really work.

Reviewed By: @mxw

Differential Revision: D1464914
Enable disabled ext_string test

Summary: Something seemed to behave differently in the interpreter vs. the jit
about this at one point, but we must've fixed it.

Reviewed By: @ptarjan

Differential Revision: D1465351
Specify ssl cipher list for curl with OpenSSL only

Summary: Since libcurl can be linked against multiple SSL libraries, it is necessary to use some light instropection when setting CURLOPT_SSL_CIPHER_LIST. This is an issue for some distributions (e.g. Fedora) where libcurl was changed to use Mozilla's NSS library. I'm not sure what cipher list would be desired for NSS, but the default seems adequate for now.

Closes https://github.com/facebook/hhvm/pull/3269

Reviewed By: @sgolemon

Differential Revision: D1455368

Pulled By: @ptarjan