Previously the generated .c file was included in another source file,
which is not supported in some build systems (e.g., meson).

Otherwise, the p11-kit-remote program called from p11-kit-server would
load the system modules instead of the local fixtures.

This enables socket activation of "p11-kit server" through systemd.
The feature provided is essentially the same as commit
a4fb2bb5 (reverted), but implemented with "p11-kit server" and
libsystemd API instead of wrapping "p11-kit remote" in the unit file.

Note that, while it exposes all tokens through the socket, it doesn't
increase attack surface beyond the PKCS#11 binary interface provided
by p11-kit-proxy.so, because the service is per-user.

Use symlink in the repository, instead of copying.

That is, use README.md as primary source to generate README as
README is required by the GNU guidelines. We don't try to convert
to "real" plain text as markdown is readable, and to avoid introducing
another dependency (e.g., pandoc).

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>