VTY: rename 'nft' to 'tunmap'

So far the config nodes were named after the implementation:
"GTP kernel module" = "gtp" and
"netfilter" = "nft"

We found that this is confusing, since both are related to handling GTP.
Rename "nft" to "tunmap"; a previous patch already renamed "gtp" to
"tunend".

Keep a hidden "nft" VTY cmd as backwards compat alias.

Related: SYS#6192
Change-Id: Ia3c5224dd3b5f5c9437bbdec997d02176818cc97

contrib/osmo-pfcp-tool-scripts/osmo-upf-11.cfg

@@ -23,5 +23,5 @@ pfcp
  local-addr 127.0.0.11
 tunend
  dev create apn11 127.0.0.11
-nft
+tunmap
  table-name osmo-upf-11

contrib/osmo-pfcp-tool-scripts/osmo-upf-12.cfg

@@ -23,5 +23,5 @@ pfcp
  local-addr 127.0.0.12
 tunend
  dev create apn12 127.0.0.12
-nft
+tunmap
  table-name osmo-upf-12