On svn-auth-x509 branch, rewrite x509parse_dn_gets() to use a svn_stringbuf_t.

* subversion/libsvn_subr/x509parse.c
  (x509parse_dn_gets): Rewrite to use svn_stringbuf_t, removing the error prone
    snprintf() calls.
  (svn_x509_parse_cert): Allocate a stringbuf for x509parse_dn_gets() to use
    and adjust the call to x509parse_dn_gets() appropriately.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1605844 13f79535-47bb-0310-9956-ffa450edef68