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.
Remove some historic comments.
Cosmetic and other small improvements to man pages.
Update packaging for libpaper 1.2

Bump the SONAME to 3 to match the bump in the sources (before, they
were out of sync).

Flesh out the changelog.
Collapse API documentation into a single man page.
Hardwire PAPERSIZE, PAPERCONF and PAPERDIR names: they are not going to change now.
Make cosmetic improvements to paperconfig and bump version.
Remove out-of-date documentation from paper.h
Clarify documentation slightly.
Remove defaultpapername API.

This function is used in some applications, but it's always redundant.
Improve the man pages.

In particular, replace repeated information by cross-references.
Improve README.
Remove papersize(5).
Tweak paperconf's argument parsing slightly to make more sense.