Various cleanups to the management of multiple TCP stacks.

Various cleanups to the management of multiple TCP stacks.

- Use strlcpy() with sizeof() instead of strncpy().

- Simplify initialization of TCP functions structures.

  init_tcp_functions() was already called before the first call to
  register a stack.  Just inline the work in the SYSINIT and remove
  the racy helper variable.  Instead, KASSERT that the rw lock is
  initialized when registering a stack.

- Protect the default stack via a direct pointer comparison.

  The default stack uses the name "freebsd" instead of "default" so
  this protection wasn't working for the default stack anyway.

Reviewed by:	rrs
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D19152