A-line:  Set TLSRVR_FRONTMOST=always if unset
tlsrvr:  Switch layers if TLSRVR_FRONTMOST=always
toolchain:  Don't pass --switch options
Merge branch 'tlsrvr-frontmost'

Use an environment variable instead of a command-line switch to indicate
that tlsrvr should bring ToolServer frontmost when running.  This allows
the user to override the toolchain default.