Adapt the Garz & Fricke naming style to the new SECO naming style.
Thus rename the gfpnmtologo to seco-pnmtologo.

LICENSE: Add the GPL-2.0 from the Linux kernel

See merge request garz-fricke/tools/gfpnmtologo!1

Add the GPL v2.0 from
https://raw.githubusercontent.com/torvalds/linux/master/LICENSES/preferred/GPL-2.0

The source code comes directly from the Linux kernel.
So also take the original version of the license from there.
It contains more than just the license Text but also the SPDX
annotations. Keep on the safe side and use the original version.

Add a parameter "-f" to define the output format. Choose between C
style source code to be used in the Linux kernel tree or a binary
representation.

Replace the global variables prefixed with "logo_" with a global
structure that resembles the kernel internal structure linux_logo.

With this it is possible to write the logo structure directly to the
binary. Add also the offset calculation for the binary into the
write_header_bin function.

Add the binary functions that write a binary based on a struct
linux_logo.

The binary will be the densely packed variant of the C-file variant.
The structure is
 +------------+-----------------------------+---------------------+
 | linux_logo |             data            |         clut        |
 +------------+-----------------------------+---------------------+
 0

The logo variable is not yet filled with data and the format_bin format
cannot be chosen from the command line.

Add the struct linux_logo from the linux kernel definition.

It is currently not used but will be used in upcoming commits.

Use the previously introduced file write abstraction and rewrite the
file writing calls accordingly.

Add the suffix "_c" to the file writing functions to make them unique
if another format will be used.
Use the renamed functions in the default output format structure
format_c.

Also use the write_clut_header function if available.

Now we should be prepared to add a binary writing format.

Add an abstraction layer for the file writing function to be able to
implement a different writing scheme.

The plan is to write a binary representation of the pnm logo that can be
directly read by the kernel. However the current implementation only
allows a C file as output. This is fine if we want to compile the logo
into the Kernel directly.

However, if we want to supply it differently, say via a prepared memory
block, we need a binary representation.

To output a binary representation we can leverage the most parsing and
clut determining code, and only the writing functions need to be
exchanged. Add the struct out_mode type to prepare for the exchange.

Add a new function write_clut_header that writes the section in between
the data and the clut part. By this we can get rid of the file writeing
calls in write_logo_clut224() in a later commit.

First yet unedited commit.

Copied most files from gfxml2dto and copied pnmtologo.c from the kernel
to gfpnmtologo.c.