All plain C examples now (mostly) adhere to the curl code style. While
they are only examples, they had diverted so much and contained all
sorts of different mixed code styles by now. Having them use a unified
style helps users and readability. Also, as they get copy-and-pasted
widely by users, making sure they're clean and nice is a good idea.
573 checksrc warnings were addressed.
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.
