Test before rename

[jheiss]

Suggested by Eric Hodel, in some cases there is a way to test a file before renaming it to the final filename. This is particularly useful for files that take effect immediately, rendering the <test_before_post> option not helpful.

The specific example Eric brought up was the sudoers file. The visudo command supports syntax testing an arbitrary filename, and the sudoers file takes effect as soon as the file is updated.

Moved from https://sourceforge.net/apps/trac/etch/ticket/18