Implement X-Sendfile2 for resume support in LigHTTPd

LigHTTPd does not support HTTP Range headers with the X-Sendfile header in the way Apache does. Instead, it needs to be handled in the backend. This commit does exactly that, using the X-Sendfile2 header to send ranges of files.
To accomplish this without breaking web servers that don't support X-Sendfile2, a new variable MOD_X_SENDFILE2_ENABLED was introduced to separate this method from X-Sendfile and X-Accel-Redirect.