Fix -s and FCGI_SOCKET_PATH env var being ignored

Patch for petr999/fcgi-spawn#1, as suggested by @proller so that specifying -s <socket> on the command line, or setting FCGI_SOCKET_PATH env var will be accepted. Otherwise the environment is completely scrubbed and the application defaults to using /tmp/spawner.sock
Merge branch 'fix_env_FCGI_SOCKET_PATH' of https://github.com/colinmkeith/fcgi-spawn into fix_env_FCGI_SOCKET_PATH