Add support for graceful handling of disk full

[Kabouik]

Might be more important now that nmail duplicates the folder currently open in temp/ when encryption is enabled, which has a significant impact on disk use when folders are big.

Also true when doing a first full-sync if nmail cache turns out to be too big.

[d99kris]

Will proceed to move all feature requests into Discussions section, for a clearer separation of bugs and feature requests.