Reworked remove() operation to separate POSIX and Windows implementations.
On Windows, if the file to be removed is read-only, try to reset the read-only
attribute before deleting the file. If deleting fails (other than because the
file is already deleted), try to restore the read-only attribute.
As a side effect, we were able to remove an implementation detail value from
the file_type enum that was used by the old remove() implementation.
Added a test for remove() on a read-only file on Windows. Also added tests
for remove_all(), including for cases with symlinks, hardlinks and read-only
files.
Also, corrected mklink /J argument in tests. The command accepts /j (lowercase)
to the same effect, but the formal help lists /J (uppercase) to create junctions.
Reported in https://github.com/boostorg/filesystem/issues/216.