[syslinux] Windows Server 2008R2 WDS workaround and pxechn.c32 status

[Gene Cumm]

As some have noticed, WDS (Windows Deployment Services) on Windows
Server 2008R2 can't normally be chained with pxechain.com.  Some have
also noticed wdsnbp.com doing something resembling another round of
DHCP (discover/ack/request/offer).  Working on pxechn.c32 I've found
the "issue" and incorporated a workaround into pxechn.c32.  wdsnbp.com
issues a DHCP discover to the IP address listed in packet #2's siaddr
field.  This normally is set to the fist boot server if no full PXE
response is received (in which case the boot server is in packet #3).
When performing a PXE restart (as with pxechain.com), normally only
packet #3 is altered.  This results in wdsnbp.com asking the wrong
server for information.

pxechn.c32 can set any DHCP option with the input as an integer parsed
by strto*(), hex bytes or a character string.  It also has a wait
option and 3 options to make adjusting the PXELINUX DHCP options
easier.

Currently, it's waiting on a function set not currently present in the
tree and final review.

-- 
-Gene