EKCD: Preload error strings, PRNG seed; use OAEP padding

Preload OpenSSL ERR string data so that the formatted error messages are
vaguely meaningful. Add OpenSSL error information to the RSA_public_encrypt()
operation failure case in one-time key generation.

For obsolescent OpenSSL versions (*cough* FIPS *cough*), pre-seed the PRNG
before entering Cap mode, as old versions of OpenSSL are unaware of kernel
RNG interfaces aside from /dev/random (such as the long-supported kern.arnd, or
the slightly more recent getentropy(3) or getrandom(2)). (RSA_public_encrypt()
wants a seeded PRNG to randomize the "PS" portion of PKCS 1.5 padding or the
"MGF" pseudo-random function in OAEP padding.)

Switch dumpon to encrypt the one-time key with OAEP padding (recommended since
1998; RFC2437) rather than the obsolescent PKCS 1.5 padding (1993; RFC2313).

Switch decryptcore to attempt OAEP decryption first, and try PKCS 1.5
decryption on failure. This is intended only for transition convenience, and
we should obsolete support for non-OAEP padding in a release or two.

Reviewed by: markj
MFC After: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D24534