`mktemp` should take mode argument

[Keno]

So permissions can be set on file creation.

[thazhemadam]

I'd like to work on this if that's alright.

I've identified

julia/base/file.jl

Line 561 in f711f0a

function mktemp(parent::AbstractString=tempdir(); cleanup::Bool=true)

and

julia/base/file.jl

Line 606 in f711f0a

function mktemp(parent::AbstractString=tempdir(); cleanup::Bool=true)

as the functions that need to be modified.

I've some idea about what can be done for non-Windows systems.
mkstemp itself will accept only char *template as its argument. So, I propose using umask to set the permissions to whatever mode specified by the user, making the ccall to mktemp, and then using umask again (only if it were called the first time around), to set the permissions to the default (600).

For Windows, however, I don't have much of an idea and would appreciate some pointers.