Teach prim_file:set_cwd() to avoid entering non-translatable directories

We have decided that we don't want to deal with the compilations
of prim_file:get_cwd() returning a binary when the current
directory name cannot be translated losslessly to a list (i.e.
when the run-time system was started with +fnu and the current directory
name contains bytes that are not part of a valid UTF-8 sequence).

Therefore, if prim_file:set_cwd() is given a binary as the pathname,
we will need to check the binary to make sure it can be translated
to a list. We will introduce a new BIF, called prim_file:is_translatable/1,
which will check both filename encoding mode, and if it is one of
Unicode modes, the binary as well.

We don't need to do anything special if prim_file:set_cwd() is passed
a list.

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