dir option must always be relative to the standard tmp directory

[silkentrance]

The dir option must be relative to the existing tmp directory that is provided by the process environment.

tmp will expect that the so defined directory is in place, otherwise it will fail as it will not create the directory by itself.

The current solution may pose issues and potential risk, see #205

The tmp directory should always and only be configured in the process environment and not by the application itself.

This might have a potential impact on existing applications and the change must be communicated well.

[silkentrance]

see also #219

[silkentrance]

having a dir option that is always relative to the os/environment provided temporary directory requires tmp to recursively create that directory, if it does not exist in order to relieve the user from doing so in every one of his applications.

we could use https://github.com/substack/node-mkdirp for that.