Fix #20925, cp not preserving permissions

[staticfloat]

Rebasing @tkelman's branch here, as I can confirm that it fixes #26907 (comment)

[staticfloat]

@vtjnash reading your comment, it appears you were asking for a mode argument. Something like this:

+"""
+    sendfile(src, dst; mode = nothing)
+
+Copy file `src` to path `dst`, creating `dst` with the given `mode`.  If `mode`
+is left at the default value of `nothing`, the mode of the `src` file is used.
+"""
+function sendfile(src::AbstractString, dst::AbstractString;
+                  mode::Union{Unsigned,Nothing}=nothing)
     src_open = false
     dst_open = false
     local src_file, dst_file
     try
         src_file = open(src, JL_O_RDONLY)
         src_open = true
-        dst_file = open(dst, JL_O_CREAT | JL_O_TRUNC | JL_O_WRONLY, filemode(src_file))
+        if mode == nothing
+            mode = filemode(src_file)
+        end
+        dst_file = open(dst, JL_O_CREAT | JL_O_TRUNC | JL_O_WRONLY, mode)
         dst_open = true

To be honest, I don't see why you would want to do this. For this to be useful, we would then add on mode kwargs to cp() and mv(), but it doesn't make sense to me why we would do that and not simply require users to chmod() after a cp() or an mv(); the fusion of these two operations doesn't make much sense to me unless you are thinking about things from the perspective of "I know that a mv from one filesystem to another is going to require an open() anyway, so I can save some work by doing it this way" in which case I would argue you should just write your own function instead of using these ones.

[vtjnash]

Mainly that open with mask typically gives a different result than chmod with mask. But I think what you're doing with this PR handles that.

[staticfloat]

Great. Then I'll assume this is an easy review for you and request one. :)

[vtjnash]

Seems like a safe enough change. In the future, we may even want to use the new, high-performance, libuv wrapper uv_fs_copyfile

[staticfloat]

Great, will merge once it's green then.

[vtjnash]

Refs nodejs/node#15394

[KristofferC]

https://travis-ci.org/JuliaLang/julia/jobs/384922807

Error in testset file:
Test Failed at /tmp/julia/share/julia/test/file.jl:91
  Expression: filemode(tmpfile) == filemode(tmpfile2)
   Evaluated: 0x000081ff == 0x000081fd
Error in testset file:
Test Failed at /tmp/julia/share/julia/test/file.jl:95
  Expression: filemode(tmpfile) == filemode(tmpfile2)
   Evaluated: 0x000081c7 == 0x000081c5
ERROR: LoadError: Test run finished with errors
in expression starting at /tmp/julia/share/julia/test/runtests.jl:59

[staticfloat]

Thanks Kristoffer, I've added umask-aware tests.

[staticfloat]

Windows failures unrelated, and we passed the file test on the windows builds anyway, so I'm calling this one good.