As reported in kern/118222, pxeboot in RELENG7 (and presumably

above) exhibits some misbehaviours on machines with AMD64 CPUs,
which at least in some cases I have tracked down to a heap overflow.

It is unclear whether it depends on the CPU or on the pxe bios
itself which may use more memory on AMD machines.

Noticeably a pxeboot compiled from 6.x sources works fine on all
machines I have tried so far, while a pxeboot compiled from 7.x
sources does not.

This patch is a first step in reducing the amount of memory used
while processing the configuration files read by the loader at boot
(some of them are quite large, 1700+ lines), and it does so by:
+ moving a buffer to static memory instead of allocating in the heap;
+ skipping empty lines;
+ reducing the amount of memory used for line descriptors;

Unfortunately there are several changes between 6.x and above,
affecting the compiler, the loader code itself, and libstand,
and it is not so straightforward to
These changes fix the behaviour on one motherboard with a
single-core AMD cpu, but are still not enough e.g on an Asus
M2N-VM (with a dual-core CPU).

I need to investigate the problem a bit more before figuring
out what should be committed to RELENG_7

PR:		kern/118222

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