Deprecated most of the old std.process functions

I have purposefully omitted doing anything about the exec* function
family here, as their fate is still under discussion.
Remove unused private writefx() function.

This eliminates the 3rd last usage of doFormat in Phobos, thus inching
us closer to fully fixing bug #4532.
Merge pull request #2403 from quickfur/issue4532b

Remove unused private writefx() function.
Merge pull request #2402 from kyllingstad/std-process-deprecated

Deprecated most of the old std.process functions