Bug#922177: ubuntu-keyring: invalid symlink if select two or more (postinst script loop loops only once (SC2066))

[Hiroyuki YAMAMORI]

Dear Maintainer,

Taking bug #922176 into account, it works well with the following code.

debian/ubuntu-keyring.postinst: line 5
```
case "$1" in
    install|configure)

      TRUSTEDPARTS="/etc/apt/tusted.gpg.d/"
      eval "$(apt-config shell TRUSTEDPARTS Dir::Etc::trustedparts/d)"

      # purge old setting before package renaming
      # from ubuntu-archive-keyring to ubuntu-keyring
      if [ -d /usr/share/doc/ubuntu-archive-keyring ]; then
        rm -f "${TRUSTEDPARTS}ubuntu-archive-keyring.gpg" \
              "${TRUSTEDPARTS}ubuntu-archive-removed-keys.gpg"
      fi

      . /usr/share/debconf/confmodule
      db_version 2.0
      db_get ubuntu-keyring/keyring

      rm -f "${TRUSTEDPARTS}ubuntu-keyring-2018-archive.gpg" \
            "${TRUSTEDPARTS}ubuntu-keyring-2012-archive.gpg" \
            "${TRUSTEDPARTS}ubuntu-archive-removed-keys.gpg"

      if [ -n "$RET" ]; then
        selected=$(echo "$RET" | sed -e 's/, /\n/g')
        echo "$selected" | while read keyring
        do
          ln -sf "/usr/share/keyrings/${keyring}.gpg" "$TRUSTEDPARTS"
        done
      fi
```

Thank you,
Hiroyuki YAMAMORI