* modules/canon-host: Add canon-host.h. Depend on getaddrinfo. Make

* modules/canon-host: Add canon-host.h.  Depend on getaddrinfo.  Make
LGPL.
* modules/getaddrinfo: Add link to opengroup spec.  Depend on strdup.
Make canon-host require getaddrinfo.
* m4/canon-host.m4 (gl_CANON_HOST): Remove most dependencies.
AC_LIBSOURCE canon-host.h.  Call...
(gl_PREREQ_CANON_HOST): ...this new function, which requires
gl_GETADDRINFO.
* m4/getaddrinfo.m4 (gl_GETADDRINFO): Compile gai_strerror when
needed.
Return usable errors from canon-host.
* lib/canon-host.h: New file.
* lib/canon-host.c (canon_host): Wrap...
(canon_host_r): ...this new function, which now relies exclusively on
getaddrinfo.
(ch_strerror): New function.
(last_cherror): New global.
* lib/getaddrinfo.c: Move include of getaddrinfo.h first to test
interface.
(getaddrinfo): Add AI_CANONNAME functionality.  Don't do arithmetic
on void *.
(freeaddrinfo): Free ai->ai_canonname when set.