What's the difference between DMD's `-preview` and `-transition`?

[Mike Franklin]

I'm planning a PR to fix 
https://issues.dlang.org/show_bug.cgi?id=6952.

The problem, as documented in issue report, is that DMD currently 
prepends `-Xlinker` to any `-L` command-line arguments (e.g `dmd 
-L-static` invokes gcc as `gcc -Xlinker -static`).  This means 
that it is impossible to pass certain gcc-specific arguments such 
as `-static`, `-nodefaultlibs`, `-nostartfiles`, etc. directly to 
`gcc`. We need to be able to invoke gcc as `gcc -static`, not 
`gcc -Xlinker - static`).

By modifying the current behavior to no longer prepend `-Xlinker` 
to `-L` command-line arguments, users will be able pass arguments 
directly to gcc (e.g. `dmd -L-static`) or simulate the current 
behavior by explicitly appending `-Xlinker` themselves (e.g. `dmd 
-L-Xlinker -L-T=script.ld`).  Both classes of users will have 
what they need.

Unfortunately, changing the current behavior could break build 
scripts that rely on `-Xlinker` being prepended to any `-L` 
command-line argument, so I need some way for users to opt-in to 
the new behavior while allowing a path forward to, someday, 
deprecate the current behavior.

For the scenario described above, should I add a 
`-preview=fixLinkerInvocation` or a 
`-transition=fixLinkerInvocation` option?

Thanks for the help.

Mike