Bug 729850 - CamelMimeParser cannot parse mbox files

This is a revert of commit f44a3d2a547f9a76d68bc63bb13eaaf165455400,
the GUnixInputStream is not seekable, not talking about portability
issues when using Unix-specific structures.
Bug 730979 - [IMAPx] Quota information is not read properly

This is regression after a fix for bug #722275.