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