Extend mktemp to support file suffixes.

[maleadt]

This PR extends mktemp to support file suffixes using the mkstemps function:

mktemp(suffix=".txt")
("/tmp/tmprFezJ1.txt",IOStream(<fd 15>))

Additionally, I've also changed the ccall signature from Ptr{UInt8} to Cstring. My understanding is that Cstring is meant to be used when a string is NULL-terminated, and according to this document the template parameter to mkstemp and others is supposed to be terminated as such.
Am I reading this correctly? If so, I think other functions (tempnam, mktempdir) might need adjusting too.

[tkelman]

needs tests, docs, and if possible a windows implementation (if not, an error)

edit: and the do-block form below with mktemp(fn, parent) should also probably be allowed to use this kwarg accordingly

[maleadt]

Updated. I force-pushed, because I reverted the change from Ptr{UInt8} to Cstring, as I'm going to keep that for another PR.

[vtjnash]

Can you explain the purpose of this PR? The Win32 API doesn't support suffixes (https://msdn.microsoft.com/en-us/library/windows/desktop/aa364991%28v=vs.85%29.aspx) so that would need to be reimplemented from scratch, and suffixes are not used much on unix systems so they should not have much value there.

[maleadt]

I was using Julia to interface with an application whose behaviour depends on the file extension, hence I needed to create temporary files with a specific suffix. Didn't seem unreasonable to me, and I had used Python's tempfile.TemporaryFile before which does support such suffixes (cpython completely reimplements this logic, not relying on OS APIs).

I botched the Windows version though, not understanding tempname creates the file on Windows. One option would be to rename the file, replacing the .tmp it is guaranteed to contain to the requested suffix. But then the file isn't guaranteed to be unique; just add a check, or really reimplement from scratch?

Or I just close this PR and don't add support for suffixes...

[tkelman]

ref #9053, still need to fix that

[hayd]

You can use do block in mktemp to avoid both close and rm.