os_xml refresh2

changes:
* new make target for coverage report of testcases
    - cd src/
    - make test
    - cd tests/
    - make generate_coverage
* xml error messages harmonized
* speedup when applying variables
    - xml array only traversed once
    - names and contents of variables are not copied
* add some testcases
    - multiple values per node (<node>first<child/>second</node>)
    - space before attribute definition (<node attr=     "value"/>)
    - comments with '!' and '-'
    - string overflow tests for xml nodes and variables