man-db: explicitly pass section string

Work around a bug in the autoconf setup of man-db: The
list of default sections does not include section 0 (zero)
despite ``./configure --help`` advertising it. This causes
header man pages (e. g. time.h from package posix_man_pages)
to be ignored by man(1):

    $ file /run/current-system/sw/share/man/man0p/time.h.0p.gz
    /run/current-system/sw/share/man/man0p/time.h.0p.gz: gzip compressed data, from Unix
    $ man 0p time.h
    No manual entry for 0p
    No manual entry for time.h

Override the default (as defined in m4/man-arg-sections.m4)
until this is fixed upstream.