import: port importd from libgcrypt to openssl^gcrypt

import: port importd from libgcrypt to openssl^gcrypt

This is heavily based on Kevin Kuehler's work, but the logic is also
significantly changed: instead of a straighforward port to openssl, both
versions of the code are kept, and at compile time we pick one or the other.

The code is purposefully kept "dumb" — the idea is that the libgcrypt codepaths
are only temporary and will be removed after everybody upgrades to openssl 3.
Thus, a separate abstraction layer is not introduced. Instead, very simple
ifdefs are used to select one or the other. If we added an abstraction layer,
we'd have to remove it again afterwards, and it don't think it makes sense to
do that for a temporary solution.

Co-authored-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
# Conflicts:
#	meson.build