Swap return value and argument of imap_threaded_capability()

Swap return value and argument of imap_threaded_capability()

imap_threaded_capability() is the only imap_threaded_*() function
which does not return status, instead status is written to a pointer
passed as argument and function returns retrieved capabilities. To
unify behaviour of all imap_threaded_*() functions make it return
status and pass capabilities through an out argument.