We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.

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

When iterating over index entries, we store the indices in an unsigned
int. As the index entrycount is a `size_t` though, this may be a loss of
precision which a compiler might rightfully complain about.

Use `size_t` instead to fix any warnings.

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.

Move `git_ref_t` to `git_reference_t`

Update some missed types that were continuing to use the old `GIT_OBJ`
names.