agent: Add option to open socket as specific user

This can prevent an attack where user A passes the path to user B's
ssh-agent socket to the daemon that is running as root.

src/libstrongswan/credentials/builder.c

@@ -20,6 +20,7 @@
 ENUM(builder_part_names, BUILD_FROM_FILE, BUILD_END,
 	"BUILD_FROM_FILE",
 	"BUILD_AGENT_SOCKET",
+	"BUILD_AGENT_USER",
 	"BUILD_BLOB",
 	"BUILD_BLOB_ASN1_DER",
 	"BUILD_BLOB_PEM",

src/libstrongswan/credentials/builder.h

@@ -49,6 +49,8 @@ enum builder_part_t {
 	BUILD_FROM_FILE,
 	/** unix socket of a ssh/pgp agent, char* */
 	BUILD_AGENT_SOCKET,
+	/** user to access a ssh/pgp agent socket, char* */
+	BUILD_AGENT_USER,
 	/** An arbitrary blob of data, chunk_t */
 	BUILD_BLOB,
 	/** DER encoded ASN.1 blob, chunk_t */