Code-cleanup TODO list

[mrc0mmand]

A couple of things that always bug me when working on dfuzzer's code

• unify comments
  • use /* */ comments everywhere, reserve // for "FIXME" cases (I'm a bit biased (not only) here by the systemd code base)
• don't require newline at the end of df_{log,verbose,debug}() messages
  • it's unnecessary clutter and df_error() already appends newline to its messages
  • also see WIP: better logging #83
• unify return codes of our functions
  • as it's currently all over the place
• get rid of goto's in main() (and maybe other places)
  • at least in main() we could make use of a global destructor for the suppression array
  • also, the first goto in main() is pointless
• possibly stop mixing glibc and glib function calls (and data types)
  • even though it should work just fine since the glibc data types are just typedefs to glibc ones (gchar -> char, etc.)
• concerning lack of Clippy in the codebase
• ...