system(gpio): add edge polling function

[gen2thomas]

Solved issues and/or description of the change

this prepares the usage of upcoming HC-SR04 driver, see #1012, independently of cdev event-handler

additionally:

• fix that all sysfs options are applied, not only the first one
• fix in sysfs that Unexport() will be called always after an export was successfully
• fix in sysf that the originated error will be returned if Unexport() was called successfully
• tests for sysfs improved

Manual test

• OS and Version (Win/Mac/Linux): Linux
• Adaptor(s) and/or driver(s):
  • raspberry
  • tinkerboard

Checklist

• The PR's target branch is 'hybridgroup:dev'
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (e.g. by run make test)
• No linter errors exist locally (e.g. by run make fmt_check)
• I have performed a self-review of my own code
• The gpio README is extended