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.

examples: modernise the fetch example

The header src/cc-compat.h defines portable format specifiers PRIuZ, PRIdZ, and PRIxZ. The original report highlighted the need to use these specifiers in examples/network/fetch.c.  For this commit, I checked all C source and header files not in deps/ and transitioned to the appropriate format specifier where appropriate.

Under normal conditions, git_remote_fetch() should be the only function
used to perform a fetch. Don't let the example lead people astray.

An anonymous remote is not configured and cannot therefore have
configured refspecs. Remove the parameter which adds this from the
constructor.