Improve the kyuafile(5) manpage
Improve the kyuafile(5) manpage

Separate concepts in subsections, add some additional details on how
test program registration works, add a synopsis and describe file
versioning.

jmmv/kyua,jmmv/kyua,jmmv/kyua,rodrigc/kyua,rodrigc/kyua,jmmv/kyua,rodrigc/kyua,rodrigc/kyua