[YSQL] Always listen on UNIX domain socket

[danielisen]

Right now YSQL only listens on the UNIX domain socket depending on the value of ysql_disable_index_backfill gflag. YW depends on YSQL always listening on this for taking YSQL backups when YSQL authentication is enabled.

The change needed is to always be listening on a UNIX domain socket regardless of the value of ysql_disable_index_backfill

[streddy-yb]

hi @daniel-yb - can you backport this to 2.4? thanks