completions: don't complete subcommands unless prev is "flux"

Problem: Unsupported subcommands in bash completions (i.e. those not
in the big case statement in _flux_core()) end up having the list
of subcommands again offered as suggestions to tab completion, e.g.
`flux perilog-run <tab>` offers the list of `flux` subcommands as
completions again. This is wrong.

Only complete subcommands when `flux` is the previous command.

etc/completions/flux.pre

@@ -2133,6 +2133,9 @@ _flux_core()
         COMPREPLY=( $(compgen -W "${cmds}" -- "$cur") )
         ;;
     *)
+        # return with no suggestions for unknown subcommands:
+        [[ "$prev" != "flux" ]] && return
+
         matched=f
         COMPREPLY=( $(compgen -W "${cmds}" -- "$cur") )
         ;;
@@ -2144,6 +2147,7 @@ _flux_core()
     if test "$matched" = "t" -a -z "$COMPREPLY" -a "$cur" = "$cmd"; then
         COMPREPLY="$cur"
     fi
+
     return 0
 }