pkcs15: new 'data' member in 'sc_pkcs15_data_info'

New member keeps the value of the PKCS#15 DATA object.

Internal pkcs15 procedure that reads DATA object is modified
to check if requested data are already vailable in 'data-info',
an only then try to read the content of dedicated on-card file.

For some emulated PKCS#15 systems value of DATA object is kept as 'direct' value
in a proprietary attribute files and so the common read procedure could not be used.

; some efforts to unify layout of code source.