Share accounts: per-site Samba passwords from the webapp

Tools > Share accounts sets the pxe-upload, blancco and clonezilla Samba
passwords. Saving one runs smbpasswd and writes it everywhere it is used:
startnet.cmd in boot.wim (pxe-upload), the served Blancco preferences and
the live config.img (blancco), the iPXE menu's Clonezilla entries
(clonezilla). Refused while a PC is imaging unless forced. Stored 0600 in
UPDATES_DIR/shares/accounts.json, never shown back.

- The stored values are the authority: the playbook only creates the
  Samba accounts, seeds the store from site-vars (pxe_upload_password,
  blancco_smb_password, clonezilla_smb_password) for accounts it does not
  hold, and runs "share_accounts apply" at the end of every run.
- Every startnet save re-pins SHAREPASS, so a restored snapshot cannot
  bring an old password back; new menu blocks use %SHAREUSER%/%SHAREPASS%.
- Shop floor unattend Order 4 reads the login from fetch-source.txt;
  Invoke-PostPpkg deletes that file once the PPKG is applied.
- Upload-Image.ps1 asks for the password unless -Pass is given.
- pxe_admin_password: the Linux pxe login. Required while the box still
  has the shipped "pxe"; set with chpasswd when it differs.
- Blancco site settings now also reach the live config.img (BIOS path).

tests/vm/README.md

@@ -70,7 +70,8 @@ Watch the server with `virt-viewer -c qemu:///system pxetest-server` (VNC on
 127.0.0.1) or screenshots. On the installed system, first-boot output goes
 to `/var/log/first-boot.log`. Once the GE NIC takes a DHCP lease on the NAT
 network, `status.sh` shows its address and `ssh pxe@<address>` works from
-the host.
+the host (password `pxetest-admin`, or `PXETEST_ADMIN_PASSWORD` given to
+build-image.sh).
 
 The offline package set must be built against the SAME ISO the image is
 built from (download-packages.sh resolves against that ISO's base). For a