maildir: hide invalid folders

The maildir worker currently populates the list of mail folders by
listing all the filesystem subdirectories in the maildir directory.
Although there's no official specification for maildir subfolders,
they should all have cur/ new/ and tmp/ subdirectories to be valid.

This patch prevents directories that don't have those subdirectories
present on the filesystem from appearing in the account folder list.

This is useful for example to prevent ".notmuch" and ".notmuch/xapian"
from showing up in the folder list if using notmuch to index emails
while using aerc's maildir backend.

Signed-off-by: Julian Pidancet <julian.pidancet@oracle.com>
Acked-by: Tim Culverhouse <tim@timculverhouse.com>

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