Format EXR: redo legacy params

1. Fix buffer overflow when 2 -> 3 *sigh*
2. Call params_size(), do not duplicate size computation
Define DT_MAX_PATH_FOR_PARAMS

WARNING: it should *NEVER* be changed!
Else old presets (created with different PATH_MAX) will break.

We can not use e.g. PATH_MAX because because it is not guaranteed
to be same across all compilers and systems.

We could use _XOPEN_PATH_MAX which is defined as 1024 bytes
in the standard - any Unix(r) or Unix-alike which claims
adherence to  IEEE Std 1003.1(2004)

But i'm not sure whether we will/will not run on systems
that do/do not claim such adherence.
Storage Disk: use DT_MAX_PATH_FOR_PARAMS to store file name

I also moved gboolean overwrite before pointer because that IMO make sense
(size = sizeof(dt_imageio_disk_t) - sizeof(void *))
Storage Email: use DT_MAX_PATH_FOR_PARAMS to store file name
Storage Gallery: use DT_MAX_PATH_FOR_PARAMS to store file name
Storage LaTeX: use DT_MAX_PATH_FOR_PARAMS to store file name
Merge pull request #682 from LebedevRI/imageio-storage-PATH_MAX

Imageio storage path max