Add an initial and work-in-progress Dutch PO translation of the FreeBSD
ocumentation Project Primer.

Some notes:

• 'make tran' generates a single book.xml from the PO file which combines the contents of the separate English files.
• Makefile is simplified to only have book.xml as source. Split output is supported as usual.
• IMAGE_LIB is left undefined, 'make' does not complain about this.