tpm2-util: rename tpm2_extend_bytes()→tpm2_pcr_extend_bytes() and make it take struct...

tpm2-util: rename tpm2_extend_bytes()→tpm2_pcr_extend_bytes() and make it take struct iovec

We soon want to add the ability to extend into nvindexes in addition to
PCRs, hence rename the function to make clear it is about pcr extension.

While we are at it, switch things over to "struct iovec" as we generally
try to do it now in tpm2-util.[ch] these days.