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Remove out-of-date documentation from paper.h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.
Make package relocatable.
Move instructions on paperspecs format into new paperspecs(5)
Remove redundant headings from paperconfig(8)
Whitespace cleanups.
Use getopt-gnu to live up to the advertised command-line syntax on less-flexible systems.
Preserve order of paper sizes in paperspecs, and try to order them commonest first.
systempapername always returns something, so don't bother checking.
Require, rather than simply attempt to find, C99 compiler.
Remove unused header.