Include <errno.h>, <stdio.h>, <stdlib.h> even

Include <errno.h>, <stdio.h>, <stdlib.h> even
if HAVE_GETLOADAVG is defined, so that the test program can work.
(errno): Remove declaration; not needed in C89 or later.
Include "c-strtod.h".
Do not include locale.h or define setlocale; no longer needed.
Include <limits.h>.
(INT_STRLEN_BOUND): New macro.
(getloadavg): Use it to compute buffer size.
Don't assume that buffer will be properly terminated by 'read'.
Use c_strtod instead of setlocale.
(main) [defined TEST]: Return int, not void.