Conditionally perform a strict check for IMAP flag availability

* elmo-imap4.el (elmo-imap4-strict-flag-availability-check): New
  variable. Perform a strict check for flag availability in set to a
  non-nil value.
 (elmo-imap4-session-flag-available-p): Use new variable and
 conditionally perform a strict check for a flag's availability.

Forcing a strict check avoids problems with IMAP server implementations
that allow the creation of user-defined flags and return an error if a
user performs a search for an undefined flag.
Merge branch 'master' of github.com:wanderlust/wanderlust