Put splicing fields into struct sosplice.
Prefix struct splice fields with sp_.
Add comment for struct sosplice.
Replace so_splice with so_sp->sp_socket.
Insert so_sp before so_splice and so_spliceback.
Add splicing defines for backwards compatibility.
Rename so_idle to o_spliceidle.
No splice in struct sosplice fields.
Add macros isspliced() and issplicedback().
Use so_sp in fill_file().
Revert "Rename so_idle to o_spliceidle."
This improves readability.

This reverts commit ddaabffe362a437e0f122b99a15fb17303483d57.

Conflicts:
	kern/uipc_socket.c
Use macros isspliced() and issplicedback(so).
Read so_splicelen in sogetopt().
Use more isspliced().
Undef the so_sp macros.
Use sb_flagsintr & SB_SPLICE in soidle().
Initialize sosplice pool.
Allocate so_sp from pool.
Return splicelen of an unspliced socket.
Use isspliced() in soreadable().