tpm2: add "systemd-tpm2-swtpm" wrapper for "swtpm"

tpm2: add "systemd-tpm2-swtpm" wrapper for "swtpm"

For TPM-less systems it's sometimes valuable to have a fill-in software
TPM running from early boot on, so that TPM-based functionality can
"just work" and rely on TPM semantics, even if it's at a substantially
weaker security level.

This adds a wrapper around swtpm. It's a binary that chainloads swtpm
but does a few preparatory steps and integrates into systemd's logic
otherwise.

All this is then exposed as systemd-tpm2-swtpm.service.

The service is not hooked into much yet, that is added in later commits.