error: 'MULTI_PERFORM_HANG_TIMEOUT' undeclared

Simplified the SMTP multi example as this example should demonstrate
the differences the easy and multi interfaces rather than introduce new
concepts such as user authentication and TLS which are shown in the TLS
and SSL examples.

Updated to read data from a callback rather than from stdio as this is
more realistic to most use cases.

Replaced the use of CURLOPT_USERPWD for the preferred CURLOPT_USERNAME
and CURLOPT_PASSWORD options and used the same username and password for
all email examples which is the same as that used in the test suite.

Updated the contents of the email and payload callback as per the IMAP
and other SMTP examples.

...and made some minor coding style changes to better match the curl
coding standards as well as the other email related examples.