Windows: factor out mutex lock/unlock from sharing.c.

This will let me reuse them easily in Pageant setup.

windows/utils/interprocess_mutex.c

@@ -0,0 +1,48 @@
+/*
+ * Lock and unlock a mutex with a globally visible name. Used to
+ * synchronise between unrelated processes, such as two
+ * connection-sharing PuTTYs deciding which will be the upstream.
+ */
+
+#include "putty.h"
+#include "security-api.h"
+
+HANDLE lock_interprocess_mutex(const char *mutexname, char **error)
+{
+    PSECURITY_DESCRIPTOR psd = NULL;
+    PACL acl = NULL;
+    HANDLE mutex = NULL;
+
+    if (!make_private_security_descriptor(MUTEX_ALL_ACCESS,
+                                          &psd, &acl, error))
+        goto out;
+
+    SECURITY_ATTRIBUTES sa;
+    memset(&sa, 0, sizeof(sa));
+    sa.nLength = sizeof(sa);
+    sa.lpSecurityDescriptor = psd;
+    sa.bInheritHandle = false;
+
+    mutex = CreateMutex(&sa, false, mutexname);
+    if (!mutex) {
+        *error = dupprintf("CreateMutex(\"%s\") failed: %s",
+                           mutexname, win_strerror(GetLastError()));
+        goto out;
+    }
+
+    WaitForSingleObject(mutex, INFINITE);
+
+  out:
+    if (psd)
+        LocalFree(psd);
+    if (acl)
+        LocalFree(acl);
+
+    return mutex;
+}
+
+void unlock_interprocess_mutex(HANDLE mutex)
+{
+    ReleaseMutex(mutex);
+    CloseHandle(mutex);
+}