Use safer allocation macros

Use of these macros, apart from the benefits mentioned in the commit
that adds the macros, has some other good side effects:

-  Consistency in getting the size of the object from sizeof(type),
   instead of a mix of sizeof(type) sometimes and sizeof(*p) other
   times.

-  More readable code: no casts, and no sizeof(), so also shorter lines
   that we don't need to cut.

-  Consistency in using array allocation calls for allocations of arrays
   of objects, even when the object size is 1.

Cc: Valentin V. Bartenev <vbartenev@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

src/id.c

@@ -23,7 +23,10 @@
 #include <pwd.h>
 #include <stdio.h>
 #include <sys/types.h>
+
+#include "alloc.h"
 #include "defines.h"
+
 /* local function prototypes */
 static void usage (void);
 
@@ -63,7 +66,7 @@ static void usage (void)
 	 * work if the system library is recompiled.
 	 */
 	sys_ngroups = sysconf (_SC_NGROUPS_MAX);
-	groups = (GETGROUPS_T *) mallocarray (sys_ngroups, sizeof (GETGROUPS_T));
+	groups = MALLOCARRAY (sys_ngroups, GETGROUPS_T);
 
 	/*
 	 * See if the -a flag has been given to print out the concurrent