build: add autotools support for the 'doc' subdir

build: add autotools support for the 'doc' subdir

'nft' documentation is originally contained in the XML file 'doc/nft.xml'.
Processing this file with the proper tools we can obtain a PDF document,
 'nft.pdf', and a unix man page, 'nft.8'.

To produce the PDF we need the tool 'dblatex' (current release
pypi.python.org/pypi/dblatex/0.3.5).

To produce the man page we use the tool 'docbook2man'; it is part of the
package 'docbook2X' (docbook2x.sourceforge.net). On some linux
distributions the tool can have slightly different names as 'docbook2x-man'
or 'db2x_docbook2man' so we search for all three names and use the first
one found and issue the command:

 # ${DB2MAN} --xinclude $<

Signed-off-by: Giorgio Dal Molin <giorgio.nicole@arcor.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>