Split content data of FCGI records to be compliant with the spec

According to the FCGI specification, the size of the content data should be
lesser or equal to 65535.
See http://www.fastcgi.com/drupal/node/6?q=node/22#S3.3 for details.
Update known dialyzer warnings
yaws_api: Fix matching of Content-Type in parse_post/1

"application/x-www-form-urlencoded" can be followed by attributes, such
as "; charset=UTF-8".
Merge pull request #180 from yakaz/fix-contenttype-matching-in-yaws_api

yaws_api: Fix matching of Content-Type in `parse_post/1`
Use iolist_to_binary/iolist_size instead of custom functions

And avoid some useless conversions