portability fixes to tools/virt-pki-validate.in

portability fixes to tools/virt-pki-validate.in

A few fixes will help make tools/virt-pki-validate.in useful on Debian
and Ubuntu.  And one fix should be useful to everyone (see #3).

 1) note our gnutls-bin package (in addition to your gnutls-utils
package) in the no-certtool error text

 2) fix a bashism, == should be = in the case where /bin/sh is a symlink
to dash

 3) $(SYSCONFDIR) cannot evaluate; set a single shell SYSCONFDIR
variable to the autoconf @SYSCONFDIR@ value, and use $SYSCONFDIR
everywhere

Bug report:
 * https://bugs.edge.launchpad.net/ubuntu/+source/libvirt/+bug/562266

Signed-off-by: Dustin Kirkland <kirkland@canonical.com>
Signed-off-by: Eric Blake <eblake@redhat.com>