os: Add Windows detection

The Windows registry key "CurrentVersion" contains both "ProductName"
as well as "BuildLab" to identify the currently running Windows
variant.

As all returned `reg` lines are either values

   name REG_<TYPE> value

or the list of subkeys, we can just use a regular expression to filter
the values from the input.

For a more generic variant, see Ashraz code on the Discord.

See also #4 for more progress on all OS detections.

eshell-info-banner.el

@@ -314,12 +314,26 @@ If RELEASE-FILE is nil, use '/etc/os-release'."
   "Read the operating system information from lsb_release"
   (shell-command-to-string "lsb_release -d -s"))
 
+(defun eshell-info-banner--get-os-information-from-registry ()
+  "Read the operating system information from the Windows registry."
+  (let ((win32-name "Windows")
+        (win32-build "Unknown"))
+    (with-temp-buffer
+      (call-process "reg" nil t nil "query" "HKLM\\Software\\Microsoft\\Windows NT\\CurrentVersion")
+      (goto-char (point-min))
+      (while (re-search-forward "\\([^[:blank:]]+\\) *\\(REG_[^[:blank:]]+\\) *\\(.+\\)" nil t)
+        (cond
+         ((string= "ProductName" (match-string 1)) (setq win32-name (match-string 3)))
+         ((string= "BuildLab" (match-string 1)) (setq win32-build (match-string 3)))))
+      (format "%s (%s)" win32-name win32-build))))
+
 (defun eshell-info-banner--get-os-information ()
   "Get operating system identifying information."
   (cond
    ((executable-find "hostnamectl") (eshell-info-banner--get-os-information-from-hostnamectl))
    ((executable-find "lsb_release") (eshell-info-banner--get-os-information-from-lsb-release))
    ((file-exists-p "/etc/os-release") (eshell-info-banner--get-os-information-from-release-file))
+   ((executable-find "reg") (eshell-info-banner--get-os-information-from-registry))
    (t "Unknown")))
 
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