feat: SystemIdentityMixin reads a host's SMBIOS identity (4.9.0)

What ties a server to its BMC: the BMC reports the server's serial
number and system UUID, the host reads the same values from its DMI
tables. Both drivers that read DMI (napalm-linux, napalm-proxmox) had a
copy of the shell for it, and neither read the board serial.

SystemIdentityMixin.get_system_identity() reads product_uuid,
product_serial and board_serial in one framed command and returns them
with vendor placeholders empty: a Supermicro board's product serial is
0123456789 on every one of them, others read "To be filled by O.E.M."
or "Default string", and many cheap boards share the UUID
03000200-0400-0500-0006-000700080009. is_placeholder_identity() is the
same filter for a BMC driver's side. A driver supplies only
_run_system_identity_command(command, privileged=); the files are
root's.

README.md

@@ -128,6 +128,15 @@ A mechanism it could not read completely is `None` ("did not look"), a host with
 `[]`. The command is about 4 kB, so a driver sends it on an exec channel
 (`_run_virtual_ips_command(command, privileged=)`), not typed into a shell.
 
+`SystemIdentityMixin` (`get_system_identity`) reads a host's SMBIOS identity -- system UUID,
+product serial and board serial from `/sys/class/dmi/id` -- which is what ties a server to
+its BMC (`ManagementControllerDriver.get_hardware_inventory` reports the same values from
+the other side). Vendor placeholders (`0123456789` on Supermicro boards, `To be filled by
+O.E.M.`, `Default string`, all-zero or the common `03000200-…-000700080009` UUID) read as
+empty: matching on them would join unrelated machines. `is_placeholder_identity` is the
+same filter for a BMC driver's side. `product_uuid` and `product_serial` are root's, so a
+driver supplies `_run_system_identity_command(command, privileged=)`.
+
 `OpenSshServerMixin` (`harden_ssh_server`, `get_ssh_server_algorithms`) takes algorithms
 out of what an OpenSSH server offers -- the ones ssh-audit flags. It reads the lists sshd
 really uses (`sshd -T`), takes out what was asked and writes the rest to