lib/tt: always escape '\' to simplify parsing in scripts

lib/tt: always escape '\' to simplify parsing in scripts

The commands echo(1) and printf(1) are usable for escape sequences
decoding, for example

 for x in $(findmnt --noheading --raw --output TARGET); do
 	printf "%b" $x
 done

but it's necessary to escape all '\' chars, otherwise for example \b
in foo\bar will be interpreted as backspace. It means that for example
findmnt(8) has to use \x5c for the backslash.

  # findmnt --noheading --raw --output TARGET /dev/sda1
  /mnt/ugly/foo\x5cbar

Reported-by: Pádraig Brady <P@draigBrady.com>
Signed-off-by: Karel Zak <kzak@redhat.com>