Make sendfile() a method in the struct fileops.  Currently only
vnode backed file descriptors have this method implemented.

Reviewed by:	kib
Sponsored by:	Nginx, Inc.
Sponsored by:	Netflix
Belatedly bump __FreeBSD_version for libc being an ld script.
This should have been done in r251668, on June 12, 2013.

This will have no practical consequences, besides having -lssp_nonshared
appearing twice on the command-line for systems built in this time frame.