Add actual querying capabilities to -query

The world is littered with broken grep commands because xrdb -query can only dump the database and doesn't implement this simple search feature.

Things I tested:

• xrdb -query without arguments works the same as before

• xrdb -query prop prints the value of prop when it exists in the resource database.

• xrdb -query prop doesn't print anything when prop doesn't exist in the resource database.

• xrdb -query prop doesn't leak any memory. I run it in valgrind with the --leak-check=full option.

PS: I think Sun took care of the fputs thing by now.