MFC 328608: Export tcp_always_keepalive for use by the Chelsio TOM module.

MFC 328608: Export tcp_always_keepalive for use by the Chelsio TOM module.

This used to work by accident with ld.bfd even though always_keepalive
was marked as static. LLD honors static more correctly, so export this
variable properly (including moving it into the tcp_* namespace).

Relative to HEAD the MFC includes two additional changes:
- The t3_tom module used for cxgb(4) is also patched.
- A strong reference from the new name (tcp_always_keepalive) to the old
  name (always_keepalive) has been added to preserve the KBI for existing
  modules.

Suggested by:	kib (strong reference)
Sponsored by:	Chelsio Communications

git-svn-id: svn://svn.freebsd.org/base/stable/10@330303 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f