Turn off Windows process ACL restriction by default.

As documented in bug 'win-process-acl-finesse', we've had enough
assorted complaints about it breaking various non-malicious pieces of
Windows process interaction (ranging from git->plink integration to
screen readers for the vision-impaired) that I think it's more
sensible to set the process back to its default level of protection.

This precaution was never a fully effective protection anyway, due to
the race condition at process startup; the only properly effective
defence would have been to prevent malware running under the same user
ID as PuTTY in the first place, so in that sense, nothing has changed.
But people who want the arguable defence-in-depth advantage of the ACL
restriction can now turn it on with the '-restrict-acl' command-line
option, and it's up to them whether they can live with the assorted
inconveniences that come with it.

In the course of this change, I've centralised a bit more of the
restriction code into winsecur.c, to avoid repeating the error
handling in multiple places.

windows/winsecur.c

@@ -224,7 +224,7 @@ int make_private_security_descriptor(DWORD permissions,
     return ret;
 }
 
-int setprocessacl(char *error)
+static int really_restrict_process_acl(char *error)
 {
     EXPLICIT_ACCESS ea[2];
     int acl_err;
@@ -287,3 +287,35 @@ int setprocessacl(char *error)
     return ret;
 }  
 #endif /* !defined NO_SECURITY */
+
+/*
+ * Lock down our process's ACL, to present an obstacle to malware
+ * trying to write into its memory. This can't be a full defence,
+ * because well timed malware could attack us before this code runs -
+ * even if it was unconditionally run at the very start of main(),
+ * which we wouldn't want to do anyway because it turns out in practie
+ * that interfering with other processes in this way has significant
+ * non-infringing uses on Windows (e.g. screen reader software).
+ *
+ * If we've been requested to do this and are unsuccessful, bomb out
+ * via modalfatalbox rather than continue in a less protected mode.
+ *
+ * This function is intentionally outside the #ifndef NO_SECURITY that
+ * covers the rest of this file, because when PuTTY is compiled
+ * without the ability to restrict its ACL, we don't want it to
+ * silently pretend to honour the instruction to do so.
+ */
+void restrict_process_acl(void)
+{
+    char *error = NULL;
+    int ret;
+
+#if !defined NO_SECURITY
+    ret = really_restrict_process_acl(error);
+#else
+    ret = FALSE;
+    error = dupstr("ACL restrictions not compiled into this binary");
+#endif
+    if (!ret)
+        modalfatalbox("Could not restrict process ACL: %s", error);
+}