Merge branch 'nm/imap-send-with-curl'

"git imap-send" has our own implementation of the protocol and also
can use more recent libCurl with the imap protocol support.  Update
the latter so that it can use the credential subsystem, and then
make it the default option to use, so that we can eventually
deprecate and remove the former.

* nm/imap-send-with-curl:
  imap-send: use curl by default when possible
  imap_send: setup_curl: retreive credentials if not set in config file
  imap-send: add wrapper to get server credentials if needed
  imap-send: return with error if curl failed