Windows text editors such as Notepad tend to use CRLF line endings,
which breaks gPXE's signature detection for script images.  Since
scripts are usually very small, they end up falling back to being
detected as valid PXE executable images (since there are no signature
checks for PXE executables).  Executing text files as x86 machine code
tends not to work well.

Fix by allowing for any isspace() character to terminate the "#!gpxe"
signature, and by ensuring that CR characters get stripped during
command line parsing.

Suggested-by: Shao Miller <Shao.Miller@yrdsb.edu.on.ca>

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

Multi-level menus via COMBOOT rely on the COMBOOT program being able
to exit and invoke a new COMBOOT program (the next menu).  This works,
but rapidly (within about five iterations) runs out of space in gPXE's
internal stack, since each new image is executed in a new function
context.

Fix by allowing tail recursion between images; an image can now
specify a replacement image for itself, and image_exec() will perform
the necessary tail recursion.

Both the script and PXE images types will claim a zero-length image.
Inhibit this to avoid end-user surprises.

executing any image, not just a script.  (This will enable pxelinux to
use relative URIs, should it wish to.)

Print error status using strerror().

allows us to avoid execution loops without having to hack around the
image registration order.

Add scripts as an image format (since it's trivial to do).