Add signelf utility

[eric_metricspace.net]

Summary

The signelf utility is a batch-signer for creating and verifying signed ELF binaries. This is a component of the proposed trust infrastructure.

Note: this review is intended to allow for commentary on the ongoing work, not to imply that this change should be committed anytime soon. It would not make much sense to commit this utility without other trust components in place.

Test Plan

This utility was tested with PKCS#7 functions in place, but these have been replaced with CMS variants. It can be tested relatively simply by signing and verifying a group of shared objects or executables.

[bcr]

Can you run textproc/igor and "mandoc -Tlint" on the man pages and see what they spit out in terms of error messages? From a quick browse, I could see a few sentences that need to have a line break after the sentence stop. These tools can tell you where.

Thanks for working on this feature!

[emaste]

@linimon I removed the "libelf" prefix; this review includes a change to libelf (which is actually the dependency D10487 and ought not be included here) but this change is not a "libelf" change.

[eric_metricspace.net]

Note, the man page for trust-config is moved to D16576