Fix storage of greetings when using IMAP_STORAGE

The store macro was not getting called preventing storage of IMAP greetings
at all. This has been corrected along with fixing checking if the
imapgreetings option is turned on to store the greeting in IMAP. Lastly,
the attachment filename was incorrectly using the full path instead of just
the basename, which was causing problems with retrieval of the greeting.

(closes issue #14950)
Reported by: noahisaac

(closes issue #15729)
Reported by: lmadsen


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@213833 65c4cc65-6c06-0410-ace0-fbb531ad65f3