Improve enums (#3168)

I realized while working on srittau/type-stub-pep#64 that a
few things we do in existing enum definitions in typeshed are
problematic:

- Using "= ..." doesn't allow type checkers to correctly type the
  result of Enum.MEMBER.value. In fact, mypy at least infers
  .value to be "Ellipsis" if you do this.
- Properties on the enum values themselves, like HTTPStatus.phrase,
  should not be specified directly as attributes, because it makes
  type checkers think that the properties themselves are enum
  members.

I ended up doing a bit more cleanup to the signal module:
- Remove unnecessary ... initializers.
- Remove unnecessary _SIG = Signals alias.
- I don't have Windows to test, but the C code for _signal suggests
  that CTRL_C_EVENT and CTRL_BREAK events are not Signals, but just ints:
  1dbd084f1f/Modules/signalmodule.c (L1575)

stdlib/2and3/socket.pyi

@@ -411,45 +411,45 @@ if sys.version_info >= (3, 4):
     from enum import IntEnum
 
     class AddressFamily(IntEnum):
-        AF_UNIX = ...
-        AF_INET = ...
-        AF_INET6 = ...
-        AF_APPLETALK = ...
-        AF_ASH = ...
-        AF_ATMPVC = ...
-        AF_ATMSVC = ...
-        AF_AX25 = ...
-        AF_BLUETOOTH = ...
-        AF_BRIDGE = ...
-        AF_DECnet = ...
-        AF_ECONET = ...
-        AF_IPX = ...
-        AF_IRDA = ...
-        AF_KEY = ...
-        AF_LLC = ...
-        AF_NETBEUI = ...
-        AF_NETLINK = ...
-        AF_NETROM = ...
-        AF_PACKET = ...
-        AF_PPPOX = ...
-        AF_ROSE = ...
-        AF_ROUTE = ...
-        AF_SECURITY = ...
-        AF_SNA = ...
-        AF_TIPC = ...
-        AF_UNSPEC = ...
-        AF_WANPIPE = ...
-        AF_X25 = ...
-        AF_LINK = ...
+        AF_UNIX: int
+        AF_INET: int
+        AF_INET6: int
+        AF_APPLETALK: int
+        AF_ASH: int
+        AF_ATMPVC: int
+        AF_ATMSVC: int
+        AF_AX25: int
+        AF_BLUETOOTH: int
+        AF_BRIDGE: int
+        AF_DECnet: int
+        AF_ECONET: int
+        AF_IPX: int
+        AF_IRDA: int
+        AF_KEY: int
+        AF_LLC: int
+        AF_NETBEUI: int
+        AF_NETLINK: int
+        AF_NETROM: int
+        AF_PACKET: int
+        AF_PPPOX: int
+        AF_ROSE: int
+        AF_ROUTE: int
+        AF_SECURITY: int
+        AF_SNA: int
+        AF_TIPC: int
+        AF_UNSPEC: int
+        AF_WANPIPE: int
+        AF_X25: int
+        AF_LINK: int
 
     class SocketKind(IntEnum):
-        SOCK_STREAM = ...
-        SOCK_DGRAM = ...
-        SOCK_RAW = ...
-        SOCK_RDM = ...
-        SOCK_SEQPACKET = ...
-        SOCK_CLOEXEC = ...
-        SOCK_NONBLOCK = ...
+        SOCK_STREAM: int
+        SOCK_DGRAM: int
+        SOCK_RAW: int
+        SOCK_RDM: int
+        SOCK_SEQPACKET: int
+        SOCK_CLOEXEC: int
+        SOCK_NONBLOCK: int
 else:
     AddressFamily = int
     SocketKind = int
@@ -458,23 +458,23 @@ if sys.version_info >= (3, 6):
     from enum import IntFlag
 
     class AddressInfo(IntFlag):
-        AI_ADDRCONFIG = ...
-        AI_ALL = ...
-        AI_CANONNAME = ...
-        AI_NUMERICHOST = ...
-        AI_NUMERICSERV = ...
-        AI_PASSIVE = ...
-        AI_V4MAPPED = ...
+        AI_ADDRCONFIG: int
+        AI_ALL: int
+        AI_CANONNAME: int
+        AI_NUMERICHOST: int
+        AI_NUMERICSERV: int
+        AI_PASSIVE: int
+        AI_V4MAPPED: int
 
     class MsgFlag(IntFlag):
-        MSG_CTRUNC = ...
-        MSG_DONTROUTE = ...
-        MSG_DONTWAIT = ...
-        MSG_EOR = ...
-        MSG_OOB = ...
-        MSG_PEEK = ...
-        MSG_TRUNC = ...
-        MSG_WAITALL = ...
+        MSG_CTRUNC: int
+        MSG_DONTROUTE: int
+        MSG_DONTWAIT: int
+        MSG_EOR: int
+        MSG_OOB: int
+        MSG_PEEK: int
+        MSG_TRUNC: int
+        MSG_WAITALL: int
 else:
     AddressInfo = int
     MsgFlag = int