Fixing ECDSA, support EdDSA signatures in CSRs.

* src/certreq.c (ksba_certreq_set_sig_val): Remove MSB handling of
0x80 for ECDSA, because it is done by GnuPG.

Distinguishing EdDSA signature in libgcrypt format, make a signature
simply concatinate r and s.

RFC 8410 specifies EdDSA signature is just concatination of R and S.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>