Fix incompatibility with versions of Perl less than 5.6.0

Fix headers_install.pl and headers_check.pl to be compatible with versions
of Perl less than 5.6.0.  It has been tested with Perl 5.005_03 and 5.8.8.
I realize this may not be an issue for most people, but there will still
be some that hit it, I imagine.  There are three basic issues:

1. Prior to 5.6.0 open() only used 2 arguments, and the versions of
the scripts in 2.6.27.1 use 3.
2. 5.6.0 also introduced the ability to use uninitialized scalar
variables as file handles, which the current scripts make use of.
3. Lastly, 5.6.0 also introduced the pragma 'use warnings'. We can use
the -w switch and be backwards compatible.

Signed-off-by: Jeremy Huntwork <jhuntwork@lightcubesolutions.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>