Mbox: Add support for reading from filehandle
Mbox: Add support for unescaping From lines
Mbox: Add function next_messageref() which returns ref to string
Mbox: Add function next_from() which returns the From_ line for next message
Mbox: Add function messageid() which returns the messageid of last read message
Mbox: Fix parsing mboxcl & mboxcl2 archives which have lot of spaces in Content-Length or Lines headers
Mbox: Add support for non-seekable filehandles