This simplifies various workflows, particularly in debugging/development.
e.g. editors will tend to propagate flags, so you can run
env CLANGD_FLAGS=-input-mirror-file=/tmp/mirror vim foo.cc rather than change
the configuration in a persistent way.
(This also gives us a generic lever when we don't know how to customize the
flags in some particular LSP client).
While here, add a test for this and other startup logging, and fix a couple
of direct writes to errs() that should have been logs.