Create global instance of command-line options

This eases testing of option values inside the code.  This instance
is implemented as the read-only copy of the obtained 'options' object,
so callers won't be able to modify its contents.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
IMAP class: don't suggest multithreading in single-threaded mode

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>