Normal evolution of make targets make it impractical to factor out
common command flags for xmlto.

The targets now list each command option as its presence or absence
needs to be justified.

xorg.ss is only needed by xmlto for html.
masterdb is only needed by xmlto.
img.src.path must not be used by html.
xsltproc need to use customization layer xorg-*.xsl.
txt format is not required to search masterdb.

Signed-off-by: Gaetan Nadon <memsize@videotron.ca>