ssh: use standalone config file for security key support

An upcoming OpenSSH update has multiple config.h settings that change
depending on whether builtin security key support is enabled. Prepare
for this by moving ENABLE_SK_INTERNAL to a new sk_config.h header
(similar to the approach used for optional krb5 support) and optionally
including that, instead of defining the macro directly from CFLAGS.

Reviewed by: kevans
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34407