hghave: enable 'serve' on Windows

I've been using a local hghaveaddon.py to enable this for a couple of months
with reasonable success, and 'killdaemons' is already enabled on Windows.
There's one failure[1] in test-http-proxy.t that this adds, which I can't figure
out.

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-April/096987.html

tests/hghave.py

@@ -502,7 +502,7 b' def has_system_sh():'
 
 @check("serve", "platform and python can manage 'hg serve -d'")
 def has_serve():
-    return os.name != 'nt' # gross approximation
+    return True
 
 @check("test-repo", "running tests from repository")
 def has_test_repo():