Bracket notation for specifying image HDU's does not work for "--flag-image-*"

[mkuemmel]

While in SE++ it is possible to give:

• --detection-image det_image.fits[1]
• --weight-image det_image.fits[2]

there is an error (file ... does not exists) when giving:

• flag-image-1 det_image.fits[3]

This is kind of inconsistent and hopefully can be fixed/repaired quickly.