MFC 354667,354686: Share routines for dealing with T6 key contexts.

354667:
Create a file to hold shared routines for dealing with T6 key contexts.

ccr(4) and TLS support in cxgbe(4) construct key contexts used by the
crypto engine in the T6. This consolidates some duplicated code for
helper functions used to build key contexts.

354686:
Add t4_keyctx.c to sys/conf/files for the non-module build.

Missed in r354667.

Sponsored by: Chelsio Communications