Installed-header hygiene (BZ#20366): conditionally defined structures.

Several network-related structures are defined conditionally under
__USE_MISC, but unconditionally used by other headers.  The path of
least resistance is usually to condition the uses on __USE_MISC as
well.

	* sysdeps/mach/hurd/net/if_ppp.h
	* sysdeps/unix/sysv/linux/net/if_ppp.h:
        Only define struct ifpppstatsreq and struct ifpppcstatsreq
        if __USE_MISC is defined, to ensure struct ifreq is declared.

	* inet/netinet/ether.h: Condition all function prototypes
        on __USE_MISC, to ensure struct ether_addr is declared.

sys/socket.h defines struct osockaddr only under __USE_MISC, whereas
protocols/talkd.h requires it unconditionally.  Here it doesn't make
sense to condition the entire body of protocols/talkd.h on __USE_MISC.
Rather than complicate sys/socket.h with a __need macro or duplicate
the definition, I am introducing a new concept: tiny headers named
bits/types/TYPE.h that define TYPE and nothing else.  This can, I hope,
ultimately replace *all* the __need macros.  The guard macro for such
headers will be __TYPE_defined, just in case application or third-party
library code is looking at them.

	* socket/bits/types/struct_osockaddr.h: New header.
	* include/bits/types/struct_osockaddr.h: New wrapper.
	* socket/Makefile: Install the new header.
	* socket/sys/socket.h,  inet/protocols/talkd.h:
	Refer to bits/types/struct_osockaddr.h for the definition of
	struct osockaddr.

socket/Makefile

@@ -23,7 +23,8 @@ subdir	:= socket
 include ../Makeconfig
 
 headers	:= sys/socket.h sys/un.h bits/sockaddr.h bits/socket.h \
-	   bits/socket2.h sys/socketvar.h net/if.h
+	   bits/socket2.h bits/types/struct_osockaddr.h \
+	   sys/socketvar.h net/if.h
 
 routines := accept bind connect getpeername getsockname getsockopt	\
 	    listen recv recvfrom recvmsg send sendmsg sendto		\

socket/bits/types/struct_osockaddr.h

@@ -0,0 +1,12 @@
+#ifndef __osockaddr_defined
+#define __osockaddr_defined 1
+
+/* This is the 4.3 BSD `struct sockaddr' format, which is used as wire
+   format in the grotty old 4.3 `talk' protocol.  */
+struct osockaddr
+{
+  unsigned short int sa_family;
+  unsigned char sa_data[14];
+};
+
+#endif

socket/sys/socket.h

@@ -38,13 +38,7 @@ __BEGIN_DECLS
 #include <bits/socket.h>
 
 #ifdef __USE_MISC
-/* This is the 4.3 BSD `struct sockaddr' format, which is used as wire
-   format in the grotty old 4.3 `talk' protocol.  */
-struct osockaddr
-  {
-    unsigned short int sa_family;
-    unsigned char sa_data[14];
-  };
+# include <bits/types/struct_osockaddr.h>
 #endif
 
 /* The following constants should be used for the second parameter of