[6.0][Networking] Search for CA roots if libcurl doesn't know where they are.

[al45tair]

Normally, distro maintainers will tell libcurl where to look when they build it, and up to now we've been relying on that. That doesn't work for the fully static Linux build, where we're building our own libcurl, and where the idea is that we'll run on any old Linux system.

To make TLS work under that circumstance, we'll need to look in a few likely places for CA root files. We only do this if libcurl doesn't already know where to look.

rdar://123434144

[al45tair]

Explanation: This is required to make Foundation Networking's SSL support work on arbitrary Linux systems; without it, libcurl only knows where to look on the system on which it was configured, which means that the fully static Linux SDK would build binaries that only worked for one system (which is annoying). Instead, we need to fall back to looking in likely places for the CA root files.
Risk: Low.
Original PR: #4877
Reviewed by: @jrflat
Resolves: rdar://123434144
Tests: Most of the new code is only fully exercised when using the fully static Linux SDK, which is presently built separately; the code paths used by normal Linux builds are tested as part of normal PR testing.

[al45tair]

@swift-ci Please test

[al45tair]

Ah, that's interesting. The macOS failure is expected (corelibs-foundation isn't used on macOS and has been failing on the main branch for a while too), but it's listed as "Required" above.

[jrflat]

I'll check with the team to see if we can get more info regarding the macOS CI.