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

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.