Enable building tar for non-FreeBSD host

For DIRDEPS_BUILD we need Makefile.depend.options to
force libegacy to be built on older FreeBSD and non-FreeBSD hosts.

Add readpassphrase to libegacy to avoid the need for libbsd on Linux

src.opts.mk disable TESTS for host if MK_host_egacy is yes

Reviewed by: imp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39766