Handle key=value format in ksadmin's CLI parser.

Chromium base::CommandLine doesn't like uppercase switches, but ksadmin
can be called with long option names using base::CommandLine instead.
However, base::CommandLine uses `key=value` format for its switches,
which ksadmin doesn't handle. ksadmin_unittest currently uses
AppendArg as a workaround, and putting switches into AppendArg is not
unique to ksadmin_unittest, but I'd rather handle AppendSwitch
correctly. (Keystone's ksadmin did.)

This CL loses the nice two-column formatting in ParseCommandLine
because clang-format kills it and generally I don't think it's worth
getting in a fight with clang-format (and the presubmit check that
complains if a file would change under clang-format).

Bug: 345863820, 341790927
Change-Id: I517339b426848cc93458da85805165bb2be505b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5709749
Reviewed-by: Xiaoling Bao <xiaolingbao@chromium.org>
Commit-Queue: Adam Norberg <norberg@google.com>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1328446}