agent: Update the key file only if not changed.
cf2d3f7ba0b7
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Description

agent: Update the key file only if not changed.

* common/name-value.c (struct name_value_container): Add flag
"modified".
(nvc_modified): New.
(nvc_new): Set flag.
(_nvc_add): Set flag.
(nvc_delete): Set flag.
(nvc_set): Set flag unless value did not change.
(nve_set): Add arg PK.  Change the caller.
* agent/findkey.c (agent_write_private_key): Update only if modified.

This helps software which uses a file system watcher to track changes
to private keys. In particular smartcard triggered changes are a
problem for such software because this may at worst trigger another
smartcard read.