Use `tpm2-software/tpm2-tss` rather than `xaptum-tpm`

[zanebeckwith]

This PR transitions this project from using xaptum-tpm for the "TSS" (TPM Software Stack), to the official tpm2-software/tpm2-tss (this is the implementation available via APT on Debian-based systems, for example).

TEMPORARY: The build is still failing, because Valgrind Memcheck is complaining about some uninitialized memory usage in the tpm2-tss library. I'll track that down (either figure out if it's our fault, or silence the warnings). But I wanted to get your feedback on the CMake stuff UPDATE: The uninitialized memory usage warning from Memcheck is happening in the tpm2-tss library's "mssim" TCTI (this is the TCTI that communicates with the software simulator we run). I checked that it doesn't arise when running against a device TPM, so it appears to be just concerning the mssim TCTI (and, since the device TCTI is OK, any production usage of this library should be OK). And, our code does properly call the "Init" function for the mssim TCTI, so this appears to be a problem in their library. I silenced the warning by explicitly zero-initializing the TCTI buffer in our tests. I'll try to track down the issue and open a PR in their repo if I can, or at least open an issue there.

This PR:

• Updates the names of constants and structs etc. as necessary to use the new API
  • Other than trivial renamings, the changes involve how the "authorizations" (e.g. password) are fed into the TSS functions. The API is actually significantly improved, and now we don't have to do a bunch of ugly pointer acrobatics)
• Adds a CMake module for "finding" the tpm2-tss libraries we need (the libecdaa-tpm library uses only the libtss2-sys library, but the tests also use either the libtss2-tcti-device or libtss2-tcti-mssim depending on whether they communicate with the TPM via a device file or via a TCP socket, i.e. with a simulator)
• Updates the travis-ci for this change
  • This means replacing the install-xaptum-tpm script with a install-tpm2-tss one
  • We also use Ubuntu 18.04 rather than 16.04 (while nice in general, the 16.04 build had issues with a too-old version of autotools failing the tpm2-tss build)
  • Note: While Ubuntu 16.04 has a libtss2-dev package available, it is very old (pre 1.x.x version), and Ubuntu 18.04 mysteriously doesn't have any libtss2 package
• While making this update, I also discovered that cppcheck was complaining about two easy-to-fix minor issues, so this PR includes those fixes, too

[coveralls]

Coverage decreased (-3.7%) to 79.902% when pulling d9b1ef7dc9d8ceeb8727bfebb3778757cbbbd1b1 on zanebeckwith/use-tpm2-tss into 15fb9ab6ef1be5cb11bb73242bf1852448ce5737 on master.

[zanebeckwith]

I just pushed two new commits, fixing two issues I found after opening this PR:

• The .travis/install-tpm2-tss script wasn't actually building the specified version, it was actually checking out the master
• The ecdaa-config.cmake file still had a find_dependency for xaptum-tpm
  • I "fixed" this by simply removing that. I wasn't sure how/whether to replace that with something analogous for the libtss2 library

[zanebeckwith]

Also, I'll update the documentation (which currently says to use a utility in xaptum-tpm to prepare a TPM for testing) after merging this

[zanebeckwith]

Also, I'll update the documentation (which currently says to use a utility in xaptum-tpm to prepare a TPM for testing) after merging this

Since this PR hadn't been merged, I just pushed these changes to this branch, instead.

It's just an update to the documentation:

• Specify tpm2-tss, not xaptum-tpm as a dependency
• Explain to use an ecdaa utility (which already existed, I assume for this purpose, but I just didn't update the README) for preparing a physical TPM for testing, rather than a utility from xaptum-tpm