Static analysis of example code found multiple findings of `printf` usage
where filling value is members of git_indexer_progress object. Specifier
used was for signed int but git_indexer_progress members are typed as
unsigned ints. `printf` specifiers were altered to match type.

Consolidate all standard includes and defines into "common.h". This lets
us avoid having to handle platform-specific things in multiple places.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Inside of our networking example code, we have a git2 executable
that acts as an entry point to all the different network
examples. As such, it is kind of the same like the normal git(1)
executable in that it simply arbitrates to the respective
subcommands.

Let's extend this approach and merge all examples into a single
standalone lg2 executable. Instead of building an executable
for all the existing examples we have, we now bundle them all
inside of the lg2 one and let them be callable via subcommands.

In the process, we can get rid of duplicated library
initialization, deinitialization and repository discovery code.
Instead of having each subcommand handle these on its own, we
simply do it inside of the single main function now.