hallib: support for system-wide halfiles

1) HALLIB_DIR: new directory for system-wide hal files (*.hal,*.tcl and
   supporting files).
   Locations:
             RIP: HALLIB = lib/hallib/
             DEB: HALLIB = /usr/share/linuxcnc/hallib/

2) HALLIB_PATH: search path for locating halfiles:
   HALLIB_PATH = .:HALLIB_DIR (e.g., ini directory then system library)

3) LIB: prefix to explicitly specify a system library halfile (with
   no search of the ini file directory)
   [HAL]
   ...
   HALFILE = LIB:filename.[hal|tcl]
   ...

This patch addresses a general issue of the multiple copies (or links) of
halfiles throughout the config tree and makes it simple to explicitly specify a
library halfile so that a user does not need to copy these files when modifying
a configuration.  For example, one can instruct a user with:
   'Include the line HALFILE = LIB:xhc-hb04.tcl in the [HAL] stanza of
    your ini file'

Signed-off-by: Dewey Garrett <dgarrett@panix.com>
docs: better "Getting LinuxCNC" intro

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>