Add Transport:sendfile/4,/5

Adds offset based sendfile to transports. Same behaviour as
file:sendfile/4,/5 except socket and file arguments are reversed and
either a raw file or a filename can be used.

sendfile/2,/4,/5 now compulsory callbacks in ranch_transport.

ranch_tcp:sendfile/2 now defaults to a chunk_size of 8191 - the default
for ranch_ssl:sendfile/2. The same default is used for both
ranch_tcp:sendfile/4,5 and ranch_ssl:sendfile/4,5.
Merge branch 'add-offset-sendfile' of git://github.com/fishcakez/ranch