Fix for dinhviethoa / libetpan Github issue #68

Modified to default to original behaviour if the servers does not
implement the CONDSTORE capability.
For servers that have CONDSTORE we use the mailimap_uid_search_modseq
function only if the search key type is one of
MAILIMAP_SEARCH_KEY_SEEN, MAILIMAP_SEARCH_KEY_ANSWERED,
MAILIMAP_SEARCH_KEY_FLAGGED, MAILIMAP_SEARCH_KEY_DELETED,
MAILIMAP_SEARCH_KEY_DRAFT,
MAILIMAP_SEARCH_KEY_MODSEQ