Using cppcheck on libgit2 sources indicated two warnings in
example code.

merge.c was reported as having a memory leak. Fix applied
was to `free()` memory pointed to by `parents`.

init.c was reported as having a null pointer dereference
on variable arg. Function 'usage' was being called with
a null variable. Changed supplied parameter to empty string.

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.

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.