Add a lens for /etc/shadow file format

and wire-up its test to the runner's list in Makefile.am

Also add a mock /etc/shadow in tests/root
Add a lens for /etc/gshadow file format

and wire-up its test to the runner's list in Makefile.am

Also add a mock /etc/gshadow in tests/root
Add a little bit of ducumentation

for both the shadow and the gshadow lenses
Refactor and document

 - import already defined lenses from the respective modules
   instead of defining them internally

 - add a missing \r to the excluded chars in sto_to_col

 - add some documentation
Merge remote-tracking branch 'origin/shadow_files' into lmc_integration
Delete a reference to YYID

the macro have been removed from bison in version 3
Merge branch 'crude_bison_workaround' into lmc_integration