libnm: belatedly expose nm_ethtool_optname_is_feature() in libnm

Also, plan right away to backport this symbol all the way back to
1.14.8. As such, we only need to add it once, with the right linker
version "libnm_1_14_8".

But still, the symbols first appears on a major release 1.20.0.

(cherry picked from commit a4c14895)
(cherry picked from commit a7322eaa)
Status Job ID Name Coverage
  Test
canceled #350888
build_f28

canceled #350889
build_f29

canceled #350890
build_f30

canceled #350891
allowed to fail
build_frawhide

canceled #350887
allowed to fail
checkpatch