Add support for path= specifier in seapp_contexts.

The path= input selector can be used to match a specific pathname
or pathname prefix (end with *) for assigning types to directories
and files within app data directories.

Change-Id: Iddaa3931cfd4ddd5b9f62cd66989e1f26553baa1
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>