agent: always use hexgrip when storing key password

The current code uses the binary ctrl->keygrip, but all the passphrase
storage engines expect this to be a string, so convert the binary
keygrip to a hex one before passing it in as the keyid. This fixes a
crash seen in some libsecret implementations where a non-ascii keyid
isn't well handled.

• Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>