search for icinga2 binary and get version

[rwaffen]

• tested only on MS Windows 2012 R2
• scan whole FS or drive C. not nice, but works.
  • this should be cached
  • could be slow

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[lbetz]

refs #705

thx, was very helpful. I initially skipped the automatic search for the binary because it was too time-consuming. The binary must be in the path or alternatively be found below the default path.