Previously there was only one "last" location stored, shared between
file and mailbox operations when the MUTT_SEL_FOLDER flags was passed.

This meant adding attachments to a composed message would start in the
"mailbox" place (which could be an IMAP folder).  Then, a subsequent
change-folder would start in the attachment directory browsed to.

Create two different flags, MUTT_SEL_MAILBOX and MUTT_SEL_FILE and use
those to use the corresponding "last" variable.

Change the browser function to use a temporary buffer, working_dir,
instead of directly manipulating LastDir.

Change callers to use the appropriate flag based on their usage.