bug#18006: Simplify via set_binary_mode

[Paul Eggert]

Eli Zaretskii wrote:
> Otherwise, the patch looks good to me.

Thanks; I installed it after fixing things up as per your comments.

Your explanation of _fmode led me to find a minor porting bug. 
Fx_load_color_file [!HAVE_X_WINDOWS] uses fopen with "rt"; unlike "rb" 
this isn't specified by POSIX and a web search suggests that it does 
fail on a few older platforms.  Emacs no longer modifies _fmode so "r" 
should suffice now anyway.  Also, there are two other places where some 
"rt"-related simplifications can be done.  Proposed further patch attached.

binary-io.patch
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