check_software: check #! in scripts installed by softwares

Some software install scripts with a #!/ line refering to a system
program, such as for example #!/usr/bin/python . Later when they are
executed this may fail because the system program might not be present
or might not be the wrong version.

This extends the software check to detect these and flag them as errors.
Software release authors will have to adjust the generated scripts, for
example by using post build step.