drbd-utils: bump version to 8.9.4

Version 8.9.4 of drbd-utils contains fixes for the musl C library.

Fixes:

  http://autobuild.buildroot.net/results/4e0/4e0ed65b2e06f4ccadcad5b4302e6334667ecbdb



Moreover, this commit adds the --without-manual option to ./configure
and drops the hook used to disable the documentation.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>