security improvement: open files with O_EXCL for writing

[vtjnash]

When opening files for writing, this package probably never wants to overwrite an existing file. This can be prevented (including any race conditions) by setting the ~O_CREAT~ O_EXCL flag when opening files for writing. This would potentially avoid any security vulnerabilities being discovered by package users resulting from malicious tar files overwriting important files.

[vtjnash]

ah, wait, sorry, the flag is O_CREAT | O_EXCL

[StefanKarpinski]

Do we have an open flag for that?

[StefanKarpinski]

Doesn't seem like we do. I guess I can use an internal open method, but maybe better to add an exclusive flag to the open function instead and then use it.