ovpn-dco: add compatibility for SUSE Linux Enterprise and openSUSE Leap

SUSE Linux Enterprise and openSUSE Leap include backported features and fixes from later releases into earlier kernels similar to how RHEL does. This commit adds support for SLE and openSUSE Leap releases beginning with SLE15 SP2 / openSUSE Leap 15.2, both released in 2020. SLE 12 SP5 uses Linux 4.12 and would require incorporating big changes in the atomics API introduced in Linux 4.19. openSUSE Tumbleweed tracks the latest upstream kernel and needs no compatibility shims.