APPS/x509: With -CA but both -CAserial and -CAcreateserial not given, use random...

APPS/x509: With -CA but both -CAserial and -CAcreateserial not given, use random serial.

Also improve openssl-x509.pod.in and error handling of load_serial() in apps.c.
Backported from https://github.com/openssl/openssl/pull/18373

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/18803)