Update information about exit status in man page

Cherry picked from commit ec0478c5ed85bec9e86d0efd5b9d24540e49b881.

[guillem@debian.org:
 - Remove internal enum names. ]

Signed-off-by: Guillem Jover <guillem@debian.org>
Rename DEBSIG_NS macro to DEBSIG_NAMESPACE

Cherry picked from commit ec0478c5ed85bec9e86d0efd5b9d24540e49b881.

Signed-off-by: Guillem Jover <guillem@debian.org>
build: Set required LFS flags to properly enable the support
debian: Extend package long description
Add a lintian override for package-contains-empty-directory
debsig-verify 0.10