Parse paperspecs at init time, not build time.

As a side-effect of using a hash table to store the paperspecs for
faster retrieval, remove the unused APIs paperprev and paperlast. None
of the Debian dependencies used these APIs.
Move remaining parts of dimen.[ch] into paper.c
Remove unused and overly-concrete APIs defaultpapersizefile and systempapersizefile
Fix expansion of autoconf variables in man pages.