also, while here, add some error checking too

replace its only usage with strncmp(). it's likely faster too.

We might end up calling client_close() from start_reply(), but that will free the fcgi/proxy bufferevent while they're still used on the stack. Instead, start_reply() only sets REQUEST_DONE and exits, returning the error eventually, so callers know when to stop.

The idea is to require SCRIPT_NAME to be defined and strip it from the beginning of the path to get PATH_INFO. Soon(tm) a `fastcgi request strip' option will be added too. Maybe even `fastcgi script name "path"` that sets SCRIPT_NAME automatically.

breaks the build with -Werror depending on the compiler.