cmd/cli: fix watching symlink /etc/resolv.conf

Currently, ctrld watches changes to /etc/resolv.conf file, then
reverting to the expected settings. However, if /etc/resolv.conf is a
symlink, changes made to the target file maynot be seen if it's not
under /etc directory.

To fix this, just evaluate the /etc/resolv.conf file before watching it.

cmd/cli/os_linux.go

@@ -24,6 +24,8 @@ import (
 	"github.com/Control-D-Inc/ctrld/internal/resolvconffile"
 )
 
+const resolvConfBackupFailedMsg = "open /etc/resolv.pre-ctrld-backup.conf: read-only file system"
+
 // allocate loopback ip
 // sudo ip a add 127.0.0.2/24 dev lo
 func allocateIP(ip string) error {