iPXE currently allocates a copy the certificate's common name as a
string.  This string is used by the TLS and CMS code to check
certificate names against an expected name, and also appears in
debugging messages.

Provide a function x509_check_name() to centralise certificate name
checking (in preparation for adding subjectAlternativeName support),
and a function x509_name() to provide a name to be used in debugging
messages, and remove the dynamically allocated string.

Signed-off-by: Michael Brown <mcb30@ipxe.org>