libsec: add X509reqtoRSApub() function and return subject alt names in X509to*pub...

libsec: add X509reqtoRSApub() function and return subject alt names in X509to*pub() name buffer

We need a way to parse a rsa certificate request and return the public
key and subject names. The new function X509reqtoRSApub() works the
same way as X509toRSApub() but on a certificate request.

We also need to support certificates that are valid for multiple domain
names (as tlshand does not support certificate selection). For this
reason, a comma separated list is returned as the certificate subject,
making it symmetric to X509rsareq() handling.

A little helper is provided with this change (auth/x5092pub) that takes
a certificate (or a certificate request when -r flag is provided) and
outputs the RSA public key in plan 9 format appended with the subject
attribute.