Replace config file parser for "xl"

Replace config file parser for "xl"

This provides a replacement config file parser for "xl" based on bison
and flex.

Benefits:
  * proper error reporting with line numbers
  * parser can understand nearly all "xm" configuration files directly
     (doesn't understand Python code but should do everything else)
  * parser also understands the ;-infested "xl" style files
  * removes the dependency on libconfig
  * better checking for certain kinds of mistakes
  * eliminates the strange "massage file and try again" code

This is intended to support all config files currently supported by
"xl" and almost all files supported by "xm".  (NB that whether a
feature works depends on the implementation of that feature in
xl/libxl of course.)

This patch also introduces a new library "libxlutil" which is mainly
for the benefit of "xl".  Users of libxl do not need to use libxlutil,
but they can do so if they want to parse "xl" files without being
"xl".

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Vincent Hanquez <vincent.hanquez@eu.citrix.com>