app_voicemail: fix bugs, imap mm_status log change to debug

Fixed some bugs:
- create dirpath when save downloading message from IMAP storage.
- create IMAP folder if not exists when saving to IMAP storage
- check if file successfully opened before write to it
- some IMAP checks
- remove non-standard flag 'Unseen'
etc

Change to debug IMAP mm_status log instead of verbose.

Remove unused X-Asterisk-VM-Caller-channel message header
for security reason. The clients should not know name of peer/endpoint.

ASTERISK-26045 #close

Change-Id: I7f83d88b69b36934e2539c114b9fb612deed971b