v4.1.x: First commit of libevent 2.1.12

[jsquyres]

I made this PR in case #10565 is not sufficient: it upgrades the libevent on the v4.1.x branch to libevent 2.1.12. I'm opening this PR to see how well it does against CI testing.

There's at least a little more work that would need to be done on this PR if we want to merge it:

1. make install-exec-hook gives warnings about common symbols in libevent .o files.
2. More extensive testing to ensure this version of libevent+its integration works in all cases.
3. Ensure that this PR does not affect things like the Open MPI v4.x ABI guarantees.
4. If libevent symbol prefixing is to be kept, the opal_rename.h file needs to be extended to include the new public symbols in libevent 2.1.12.

bot:notacherrypick

Refs: #10542, #10565, #10583

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Replace libevent 2.0.22 with 2.1.12 to address some CVEs. The CVEs in question were in code that Open MPI did not use, and were therefore harmless. However, in order to not have to continually explain this to end users, we'll just upgrade the embedded libevent to a version that does not contain the CVEs at all.

Signed-off-by: Jeff Squyres [email protected]

[jsquyres]

Hey @bwbarrett I'm unable to replicate the linker error that is showing up on the OMPI/Jenkins CI link fails. I've tried on MacOS, RHEL 6 and 7, and Ubuntu 20 (which is what the CI is using). Any idea what is happening on that failure?

[jsquyres]

After much discussion and back-n-forth, this PR seems to be too much of a pain. Instead, we're going to merge the upstream patches in #10565. Closing this PR.