Previously, mpv.exe used the --terminal option to decide whether to
attach to the parent process's console, which made it impossible to tell
whether mpv would attach to the console before the config files were
parsed. Instead, make mpv always attach to the console when launched
from the console wrapper (mpv.com) and never attach otherwise. This will
be useful for the next commit, which will use the presence of the
console to decide whether to use the pseudo-gui profile.
This change should also be an improvement in behavior. The old code
would attach to the parent process's console, regardless of whether it
was mpv.com or some other program like cmd.exe. This could be confusing,
since mpv.exe is marked as a Windows GUI program and shouldn't write
text to its parent process's console when launched directly. (See #768.)
Visual Studio does something similar with its devenv.com wrapper.
devenv.exe only attaches to the console when launched from devenv.com.
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