Convert the documentation PS to PDF and include again in tarballs

The pdf is about 1/6 of the size with no quality loss that I can tell.
It's actual professionally written documentation even if a bit outdated,
might as well include it. Also fixup the reference in the README.

Makefile.am

@@ -2,7 +2,7 @@
 
 MCCABE =	pmccabe
 
-EXTRA_DIST = autogen.sh CHANGES $(man_MANS) ci/Dockerfile build-aux
+EXTRA_DIST = autogen.sh CHANGES $(man_MANS) ci/Dockerfile build-aux popt.pdf
 
 SUBDIRS = src po auto tests
 

README

@@ -8,7 +8,7 @@ to getopt(3), it contains a number of enhancements, including:
 	4) popt provides convience functions for parsing strings
 	   into argv[] style arrays
 
-Complete documentation on popt(3) is available in popt.ps (included in this
+Complete documentation on popt(3) is available in popt.pdf (included in this
 tarball), which is excerpted with permission from the book "Linux
 Application Development" by Michael K. Johnson and Erik Troan (available
 from Addison Wesley in May, 1998).