Add license text to fdup.1

This way, I comply with my own license which requires distributors of binary
code to replicate the license text inside the documentation.
Rework source tarball rule

There is a target tarball now that will automatically generate a tarball with
a suitable name. Also, we use git describe to generate a revision name now.
This is far more reliable than git rev-parse as it automatically incorporates
the name of tags.
Move source files into src directory
Do not put git specific files in tarballs
Add installation instructions