sctp: bundle usrsctp

sctp: import internal copy of usrsctp library
There are problems with global shared state and no API stability
guarantees, and we can't rely on distros shipping the fixes we
need. Both firefox and Chrome bundle their own copies too.

Imported from https://github.com/sctplab/usrsctp,
commit 547d3b46c64876c0336b9eef297fda58dbe1adaf
Date: Thu Jul 23 21:49:32 2020 +0200

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/issues/870

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sctp: hook up internal copy of libusrsctp to build
Add option 'sctp-internal-usrsctp' so people can choose
to build againts the distro version instead.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/issues/870

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Compiler warnings need reviewing (and/or suppressing) to make sure things are hooked up right.

I also haven't tested this yet besides the basic build.

Todo

• test
• figure out how to exempt those files or dirs from gst-indent in local pre-commit hook, don't want to add indent on/off markers to all these files (removed for now, can just use git commit -n for time being)
• make gst indent job on CI honour the .indentignore file: gst-ci!328 (merged)