Sanitize environment before starting backend processes (rclone, ssh)

The restic security model includes full trust of the local machine, so
this should not fix any actual security problems, but it's better to be
safe than sorry.

Fixes #2192.

internal/backend/foreground_unix.go

@@ -24,10 +24,7 @@ func tcsetpgrp(fd int, pid int) error {
 	return errno
 }
 
-// StartForeground runs cmd in the foreground, by temporarily switching to the
-// new process group created for cmd. The returned function `bg` switches back
-// to the previous process group.
-func StartForeground(cmd *exec.Cmd) (bg func() error, err error) {
+func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
 	// open the TTY, we need the file descriptor
 	tty, err := os.OpenFile("/dev/tty", os.O_RDWR, 0)
 	if err != nil {