include: fix SYS_{unshare,nsenter} usage

Signed-off-by: Karel Zak <kzak@redhat.com>
build-sys: fix nsenter requirements

Signed-off-by: Karel Zak <kzak@redhat.com>
sulogin: fix compiler warnings

Signed-off-by: Karel Zak <kzak@redhat.com>
fdisks: fix compiler warning

Signed-off-by: Karel Zak <kzak@redhat.com>