Add global and system-wide gitattributes

Add global and system-wide gitattributes

Allow gitattributes to be set globally and system wide. This way, settings
for particular file types can be set in one place and apply for all user's
repositories.

The location of system-wide attributes file is $(prefix)/etc/gitattributes.
The location of the global file can be configured by setting
core.attributesfile.

Some parts of the code were copied from the implementation of the same
functionality in config.c.

Signed-off-by: Petr Onderka <gsvick@gmail.com>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>