ecc: Fix ECDH of Curve25519.
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ecc: Fix ECDH of Curve25519.

* cipher/ecc-misc.c (_gcry_ecc_mont_decodepoint): Fix calc of NBITS
and prefix detection.
* cipher/ecc.c (ecc_generate): Use NBITS instead of CTX->NBITS.
(ecc_encrypt_raw): Use NBITS from curve instead of from P.
Fix rawmpilen calculation.
(ecc_decrypt_raw): Likewise.  Add debug output.

This fixes the commit dd3d06e7. NBITS is defined 256 in ecc-curves.c,
thus, ecc_get_nbits returns 256. But CTX->NBITS has 255 for Montgomery
curve.