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.

Recipe

@@ -54,8 +54,7 @@
 #         security grounds (although it will run fine on Win95-series
 #         OSes where there is no access control anyway).
 #       - SSH connection sharing is disabled.
-#       - There is no restriction of the process ACLs (on all versions
-#         of Windows, without warning), as if UNPROTECT below were set.
+#       - There is no support for restriction of the process ACLs.
 #
 #  - COMPAT=/DNO_MULTIMON (Windows only)
 #      Disables PuTTY's use of <multimon.h>, which is not available
@@ -108,10 +107,6 @@
 #  - XFLAGS=/DDEBUG
 #      Causes PuTTY to enable internal debugging.
 #
-#  - XFLAGS=/DUNPROTECT
-#      Disable tightened ACL on PuTTY process so that e.g. debuggers
-#      can attach to it.
-#
 #  - XFLAGS=/DMALLOC_LOG
 #      Causes PuTTY to emit a file called putty_mem.log, logging every
 #      memory allocation and free, so you can track memory leaks.