The parameters of the `git_pkt_parse_line` function are quite confusing.
First, there is no real indicator what the `out` parameter is actually
all about, and it's not really clear what the `bufflen` parameter refers
to. Reorder and rename the parameters to make this more obvious.

When parsing data, progress or error packets, we need to copy the
contents of the rest of the current packet line into the flex-array of
the parsed packet. To keep track of this array's length, we then assign
the remaining length of the packet line to the structure. We do have a
mismatch of types here, as the structure's `len` field is a signed
integer, while the length that we are assigning has type `size_t`.

On nearly all platforms, this shouldn't pose any problems at all. The
line length can at most be 16^4, as the line's length is being encoded
by exactly four hex digits. But on a platforms with 16 bit integers,
this assignment could cause an overflow. While such platforms will
probably only exist in the embedded ecosystem, we still want to avoid
this potential overflow. Thus, we now simply change the structure's
`len` member to be of type `size_t` to avoid any integer promotion.

use consistent names for the #include / #define header guard pattern.

Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.

This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.

This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.

This change provides an option in git_libgit2_opt_t which can be used in git_libgit2_opts to turn off/on ofs_delta capability in libGit2