Allow native POP3 retrievers without an IMAP mailbox list. Preserve
explicit mailbox defaults and cover the omitted setting in the account
configuration test.
Expose native filter, retriever, and destination configuration while
retaining account defaults and legacy boolean aliases. Preserve tuple
rendering and allow complete native retriever configuration without an
IMAP account block.
The mailboxes must be a tuple of string or the string "ALL".
The generated value was broken if the mailboxes configuration was a list
of only one string (but not "ALL"): the generated expression ( "str" )
was not a tuple but a string.
Now, we always generate a tuple (by adding a comma, even with a list of
size one). Getmail works with the special value "ALL" whether it is a
in tuple or not, so this case is not specifically handled.
Before the XDG variables would be set from the user's environment, if
available. This would break some tests.
With this change the tests should be fully deterministic.
Fixes#1190
