pkcs11-tool sets CKA_DECRYPT=true rather then CKA_DERIVE=true when generating EC keys (#277)

RSA and EC keys have different usage attributes. Appropriate attributes are set
When using --keypairgen the user can use the --usage-sign, --usage-decrypt,
and --usage-derive. to get finer control.

 Changes to be committed:
	modified:   tools/pkcs11-tool.c