Improve "keytool -printcert -jarfile" output

A few improvements to the output would help readability and diagnosis:

1. Remove "Signature:" from output. It is printing information about certificates, and not the signature.

2. Add the certificate # in the output so it is easier to figure out which certificate a warning applies to. For example:

Signer #1:

Certificate #1:

3. Add the signer # to the warning output, so it is easier to figure out which signer it applies to. Ex, change:

"The certificate #3 of 3 uses the SHA1withRSA signature algorithm ..."

to:

"The certificate #3 of 3 of signer #1 uses the SHA1withRSA signature algorithm ..."