debian: Fix FTBFS problem caused by texi2html changing its output location

debian: Fix FTBFS problem caused by texi2html changing its output location

Unfortunately, texi2html gratuitously changed its behavior of where
its output html files are placed when the -split_chapter is in effect.
(First it was in a subdirectory; then it was in the current directory;
now it's back to putting the output html files in a subdirectory
again.)

Support either way of doing things since the texi2html team seems to
be indecisive...

Addresses-Debian-Bug: #552934

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>