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.

This fixes two markup typos I noticed in curl_easy_setopt.3.  (The use
of bold vs. italics seems a bit inconsistent in that page, but it should
at least be valid man syntax.)

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.

Although added to CURLOPT_INFILESIZE in commit ee3d3adc it was
never added to CURLOPT_INFILESIZE_LARGE.

Make sure that we detect such attempts and return a proper error code
instead of silently handling this in problematic ways.

Updated the documentation to mention this limitation.

Bug: http://curl.haxx.se/bug/view.cgi?id=1286