preconv is now on encoding-recognition parity with groff. This last

preconv is now on encoding-recognition parity with groff.  This last
commit adds parsing of "File Variables" in the first two lines in order
to grok the encoding.  This completes groff's recognition sequence (-e,
BOM, File variables, -D, default).  I've also cleaned up the manual to
indicate this and for some general readability.

preconv is now compiled by default in the Makefile.