pxe: disentangle the legacy and lwip stacks without #ifdef
Disentangle the dependencies via the legacy and the lwip stacks. Now
there are no more #ifdefs (except for a single one in pxelinux.asm)
and the only difference between the pxelinux and lpxelinux archives
are which files are included.
To make this happen:
1. Replace the private part of the socket structure with a union.
2. Move the list of URL methods into a stack-dependent file.
Reported-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>