trace: allow writing traces to a socket

Git can write trace2 events to a Unix domain socket [1]. This can be
specified via Git's `trace2.eventTarget` config option, which we read to
determine where to log our own trace2 events. Currently, if the Git
config specifies a socket as the trace2 target, we fail to log any
traces.

Fix this by adding support for writing to a Unix domain socket,
following the same specification that Git supports.

[1]: https://git-scm.com/docs/api-trace2#_enabling_a_target

Change-Id: I928bc22ba04fba603a9132eb055141845fa48ab2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/332339
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Josh Steadmon <steadmon@google.com>