Fix building against LibreSSL

[klemensn]

With the upcoming 3.6 release LibreSSL will ship
X509_VERIFY_PARAM_get_time() as required by libdigidocpp.

Furthermore, the following changes are required to build latest HEAD against
current LibreSSL:

• Omit SHA3 with LibreSSL
• Omit BIO_F_BIO_LOOKUP_EX handling with LibreSSL
• Provide OPENSSL_memdup() and TS accessors with LibreSSL
• Use old-style STACK_OF stuff for LibreSSL
• Do not define ESS_ISSUER_SERIAL with LibreSSL

All patches patches were written by @botovq against the 3.14.8 release during
review of my OpenBSD ports/package submission for libdigidocpp[0].

I rebased the patches onto latest HEAD (d9fd053) and retested on
OpenBSD/amd64 7.1 -CURRENT, i.e. today's latest LibreSSL development version.

0: openbsd/ports@a015763

[klemensn]

This PR is a good start, but signature validation is still broken with LibreSSL.

v3.14.8 built against OpenSSL on OpenBSD works:

$ digidoc-tool create --file=/tmp/hello.txt /tmp/hello.asice
Version
  digidoc-tool version: 3.14.8.0
  libdigidocpp version: 3.14.8.0
Available certificates:
  label: ...
Selected:
  label: ...
Please enter PIN for token '...' or <enter> to cancel: 
    Validation: OK

but both v3.14.8 and latest HEAD (d9fd053) built with this PR fail with

    Validation: FAILED
     Exception:
SignatureXAdES_LTA.cpp:203 code(General) Signature validation
TS.cpp:288 code(General) Failed to verify TS response.

[klemensn]

This PR is a good start, but signature validation is still broken with LibreSSL.

This was due to lack of RFC 5035 support in LibreSSL.

I added the missing pieces such that latest libdigidocpp HEAD (06a755d) can now successfully validate documents on OpenBSD/LibreSSL (with uncommitted changes):

$ LD_LIBRARY_PATH=$PWD/obj digidoc-tool sign /tmp/hello.asice
Version
  digidoc-tool version: 3.14.10.0
  libdigidocpp version: 3.14.10.0
Available certificates:
  label: ...
Selected:
  label: ...
Please enter PIN for token '...' or <enter> to cancel: 
    Validation: OK

[klemensn]

Latest LibreSSL is now able to validate libdigidocpp's timestamps: openbsd/src@9c1c430

[klemensn]

Updated with the following:

• size 0 yields NULL as per OpenSSL's CRYPTO_malloc() semantics[0],
  so treat size == 0 in the added OPENSSL_memdup() accordingly

• LibreSSL has ASN1_STRING_get0_data(3), so no need to redefine it.

0: https://github.com/openssl/openssl/blob/OpenSSL_1_1_1-stable/crypto/mem.c#L200-L201