* lib/xvasprintf.h: Clarify the programmer's responsibilities.
(xasprintf, xvasprintf): Declare as returning non-NULL.
* lib/xvasprintf.c: Include <stdlib.h>.
(xstrcat): Allow results longer than INT_MAX characters. Upon size
overflow, invoke xalloc_die.
(xvasprintf): Call vazsprintf instead of vasprintf. When some other
error occurs, emit an error message and abort.
* m4/strerrorname_np.m4 (gl_CHECK_STRERRORNAME_NP): New macro, extracted
from gl_FUNC_STRERRORNAME_NP.
(gl_FUNC_STRERRORNAME_NP): Invoke it.
(gl_OPTIONAL_STRERRORNAME_NP): New macro.
* m4/xvasprintf.m4 (gl_XVASPRINTF): Invoke gl_OPTIONAL_STRERRORNAME_NP.
* modules/xvasprintf (Files): Add m4/strerrorname_np.m4.
(Depends-on): Add extensions, vazsprintf. Remove vasprintf.
* NEWS: Mention the change.