Fix TARGET_APPIMAGE issues

[enzo1982]

I discovered two issues with TARGET_APPIMAGE while working on #126:

• --appimage-help is not working when TARGET_APPIMAGE is used
• --appimage-portable-* is not working when TARGET_APPIMAGE is used

Both are due to readlink being used in different places without checking that appimage_path is a link.

This PR fixes this by moving the code to get the full AppImage path up and adding a check to see whether appimage_path is a link.

[TheAssassin]

Why would you want to use readlink at all? You can also open a symlink.

[enzo1982]

Why would you want to use readlink at all? You can also open a symlink.

--appimage-help and --appimage-portable-* need to know the AppImage name in order to print it/create a folder with that name. appimage_path usually is /proc/self/exe, so that link needs to be resolved.

[TheAssassin]

I see, I thought that was likely already handled. Mind to add comments in the source code with your motivation to avoid people from introducing regressions?