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.

Right now, we have two sets of "common" code, one containing
general common code and one containing network common code. As we
intend to get rid of the network subdirectory and instead merge
all examples into a single standalone executable, this
distinction doesn't make a lot of sense now. Furthermore, the
common network code is not that big.

Let's get rid of the common network code by merging it into the
general common code.

Pack file verification

C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.

We strive to keep an options structure to many functions to be able to
extend options in the future without breaking the API. `git_indexer_new`
doesn't have one right now, but we want to be able to add an option
for enabling strict packfile verification.

Add a new `git_indexer_options` structure and adjust callers to use
that.

It was there to keep it apart from the one which read in from a file on
disk. This other indexer does not exist anymore, so there is no need for
anything other than git_indexer to refer to it.

While here, rename _add() function to _append() and _finalize() to
_commit(). The former change is cosmetic, while the latter avoids
talking about "finalizing", which OO languages use to mean something
completely different.

When given an ODB from which to read objects, the indexer will attempt
to inject the missing bases at the end of the pack and update the
header and trailer to reflect the new contents.