UserInput and ConfigFile: improved string conversions

   - more strict checking of INT8, INT4, REAL8, and BOOLEAN string
     conversions: don't allow any trailing garbage
   - Unify string-conversions between UserInput and ConfigFile modules
     using new API functions XLALParseStringValueTo{INT8|INT4|REAL8|BOOLEAN}()
   - some minor documentation and coding-style improvements
   - passes 'make distcheck'
   - refs #1094