Redo the whole doc/ build thing

You can now use: make {all-docs,pdf,html,man,api,update-man}
     instead of: make {all-docs,pdf-docs,html-docs,man-docs}

Away is building into separate dirs. This makes make able to actually get
dependencies right, since there are now a collection of 'common' build
rules, some of which have even been moved to use the cmd infrastructure.

To update the man pages there is a new update-man rule. It builds the
manual pages and copies them to their proper place under man/ while doing
the final preformatting.

As good thing is that the (two) man pages are moved to .txt files and
include the generated content.

The API building thing is also refined. It builds into api/ and builds it's
list of files dynamically by searching throught the .h files in the src/
directory. Documented header files must contain a comment like this:

	/* API Doc :: <api-name> */

where <api-name> is the name used for the file under api/, for example
dom-scanner.

doc/installation.txt

@@ -71,6 +71,6 @@ include::small.txt[]
 
 include::ecmascript.txt[]
 
-include::txt/import-features.conf.txt[]
+include::import-features.conf.txt[]
 
 endif::installation-webpage[]