gnutls-serv: allow user to specify multiple x509certile/x509keyfile
Instead of adding more and more variants like x509dsakeyfile or x509ecckeyfile (counting rsa-pss and gost in future), allow user to specify x509certfile/x509keyfile multiple times.
Signed-off-by: Dmitry Eremin-Solenikov dbaryshkov@gmail.com