In the maildir worker we manually need to track the Recent flag in order for the
notification command etc to work.
Push that responsibility to the container, we must make sure to manually add the
flag though if one grabs the message info.
If accounts.conf contains an invalid maildir url, return a nice
error instead of panicking.
Log a couple of different error cases to provide extra
information about the error to the user.
Add the initial implementation of a backend for Maildir accounts. Much
of the functionality required is implemented in the go-message and
go-maildir libraries, so we use them as much as possible.
The maildir worker hooks into a new maildir:// URL scheme in the
accounts.conf file which points to a container of several maildir
directories. From there, the OpenDirectory, FetchDirectoryContents, etc
messages work on subdirectories. This is implemented as a Container
struct which handles mapping between the symbolic email folder names and
UIDs to the concrete directories and file names.
