linuxcnc.in: support halfiles in a system lib

This patch creates a system directory for halfiles so that a user can specify
using a new prefix (LIB:) as:
  [HAL]
  ...
  HALFILE = LIB:filename.[hal | tcl]
  ...

(next step, notdoneyet: the patch likely needs testing and work for
deb package building)
RIP: lib/hallib/
DEB: /usr/share/linuxcnc/hallib/

This patch addreses a general problem of multiple copies (or links)
of halfiles throughout the config tree and communicating with users
with little linux experience and variations in locations for
RIP and deb packages.

Recently motivatd by:
1) When working on moveoff, i include a halfile (hookup_moveoff.tcl)
   in configs/sim and in some moveoff_demo*.ini files
   (it is softlinked from configs/sim/axis, configs/sim/touchy/ngcgui)

2) Instructions for a user to modify an existing sim:
   'copy from configs/sim/hookup_moveoff.tcl to your config directory
    and then included it in your inifile [HAL]HALFILE=hookup_moveoff.tcl'

so it would be much simpler to say:
   'add an entry in the [HAL] stanza:
   HALFILE = LIB:hookup_moveoff.tcl

The use of [HAL]HALFILE = LIB:filename could be used in numberous places.

This patch is for test and demonstration only.

Signed-off-by: Dewey Garrett <dgarrett@panix.com>