Add support for git attribute macro definitions.  Also, add
support for cache flush API to clear the attribute file content
cache when needed.

Additionally, improved the handling of global and system files,
making common utility functions in fileops and converting config
and attr to both use the common functions.

Adds a bunch more tests and fixed some memory leaks.  Note that
adding macros required me to use refcounted attribute assignment
definitions, which complicated, but probably improved memory usage.