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