Kevin suggested doing the reverse of my last commit, since imap_retrieve_file

Kevin suggested doing the reverse of my last commit, since imap_retrieve_file
does not modify the contents of the "mailbox" string. In other words, I'm changing
the imap_retrieve_file function to take a const char* as the third argument so that I
don't need to cast const char*'s as char*'s to suppress compiler warnings.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@90930 65c4cc65-6c06-0410-ace0-fbb531ad65f3