Partially address JuliaLang#7385

This does not fully work on Windows (no support for ACLs), but is no worse than the current situation

Author: ihnorton

base/stat.jl

@@ -96,7 +96,6 @@ for f in {
     :issetgid
     :issticky
     :isreadable
-    :iswritable
     :isexecutable
     :uperm
     :gperm
@@ -107,6 +106,8 @@ end
 
 islink(path...) = islink(lstat(path...))
 
+@windows_only iswritable(path...) = ccall(:_waccess, Cint, (Ptr{Uint16}, Cint), utf16(joinpath(path...)), 2) == 0
+@unix_only    iswritable(path...) = ccall(:access, Cint, (Ptr{Uint8}, Cint), joinpath(path...), 2) == 0
 
 # some convenience functions