- Separate sendfile(2) implementation from uipc_syscalls.c into

separate file.  Claim my copyright.
- Provide more comments, better function and structure names.
- Sort out unneeded includes from resulting two files.

No functional changes.

sys/conf/files

@@ -3213,6 +3213,7 @@ kern/kern_rmlock.c		standard
 kern/kern_rwlock.c		standard
 kern/kern_sdt.c			optional kdtrace_hooks
 kern/kern_sema.c		standard
+kern/kern_sendfile.c		standard
 kern/kern_sharedpage.c		standard
 kern/kern_shutdown.c		standard
 kern/kern_sig.c			standard