buildsys: Check for mkstemps()

This function is defined in Bionic, but it is missing from the headers.
Let's just ignore it for now.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
cli: Add --qr and --show-qr export options

These just invoke external utilities (qrencode and image viewers) but
it is more convenient than remembering the long-ish command lines needed
to convert a token into a QR code.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
man: Update documentation

Update QR info; fix formatting and TODO list.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>