Move linux specific cmake files into linux subdirectory.
Adjust paths to files.
Move platform dependent files into linux subdirectory.
Copy config header file into build directory when running cmake.
Make cmake commands upper case.
Adjust paths to make unit tests building after moving them into the linux subdirectory.
Simplify some conditions and make some variables unsigned.

This is mainly done to avoid overflows of signed integers, which
is not defined.
Add missing include.
Add comment to TODO list.
Add multithreaded implementation of I/O handling.
Start the client threads in detached state.
Modify echo client to measure round trip times for several message sizes.
Remove empty line.
Flush stdout after writing a line.
Move some initialization code to make it more similar to the code in main_mt.c
Remove unecessary goto statement.
Move code to separate IO specific stuff.
Move IO specific stuff into separate files.
Take out op parameter of add_epoll().

It's just not required.
Choose IO model with define in config.h