In preparation of merging projects/sendfile, transform bare access to
sb_cc member of struct sockbuf to a couple of inline functions:

sbavail() and sbused()

Right now they are equal, but once notion of "not ready socket buffer data",
will be checked in, they are going to be different.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Merge from projects/sendfile:
- Use KASSERT()s instead of panic().
- Use sbavail() instead of sb_cc.

Sponsored by:	Nginx, Inc.
Sponsored by:	Netflix