Edit piuparts_slave_* man pages for more information and format.
Specify the manpage doctype for AsciiDoc man sources.

Otherwise, e.g. 'a2x -f pdf <manpage>.txt' yields a document in book
format, with a title page and table of contents.
bump Build-Depends to python (>= 2.7)

Signed-off-by: Andreas Beckmann <anbe@debian.org>
Makefile: don't re-enumerate files

Signed-off-by: Andreas Beckmann <anbe@debian.org>