Security update for libressl (moderate)

This update for libressl to version 2.8.0 fixes the following issues:

Security issues fixed:

• CVE-2018-12434: Avoid a timing side-channel leak when generating DSA and
  ECDSA signatures. (boo#1097779)
• Reject excessively large primes in DH key generation.

Other bugs fixed:

• Fixed a pair of 20+ year-old bugs in X509_NAME_add_entry.
• Tighten up checks for various X509_VERIFY_PARAM functions, ‘poisoning’
  parameters so that an unverified certificate cannot be used if it fails
  verification.
• Fixed a potential memory leak on failure in ASN1_item_digest.
• Fixed a potential memory alignment crash in asn1_item_combine_free.
• Removed unused SSL3_FLAGS_DELAY_CLIENT_FINISHED and
  SSL3_FLAGS_POP_BUFFER flags in write path, simplifying IO paths.
• Removed SSL_OP_TLS_ROLLBACK_BUG buggy client workarounds.
• Added const annotations to many existing APIs from OpenSSL, making
  interoperability easier for downstream applications.
• Added a missing bounds check in c2i_ASN1_BIT_STRING.
• Removed three remaining single DES cipher suites.
• Fixed a potential leak/incorrect return value in DSA signature
  generation.
• Added a blinding value when generating DSA and ECDSA signatures, in
  order to reduce the possibility of a side-channel attack leaking the
  private key.
• Added ECC constant time scalar multiplication support.
• Revised the implementation of RSASSA-PKCS1-v1_5 to match the
  specification in RFC 8017.