mallinfo-util: assume mallinfo() exists

The check existed for musl. Let's remove it, as we explicitly request glibc.

While removing the check, this also drops generic_mallinfo, introduces
a tiny converter from struct mallinfo to struct mallinfo2 if mallinfo2()
does not exist, and renames mallinfo-util.h to malloc.h.

With this change, we can drop many ifdefs and casts in .c files.

src/basic/include/malloc.h

@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include_next <malloc.h>
+
+#include "macro.h"
+
+#if !HAVE_MALLINFO2
+struct mallinfo2 {
+        size_t arena;    /* non-mmapped space allocated from system */
+        size_t ordblks;  /* number of free chunks */
+        size_t smblks;   /* number of fastbin blocks */
+        size_t hblks;    /* number of mmapped regions */
+        size_t hblkhd;   /* space in mmapped regions */
+        size_t usmblks;  /* always 0, preserved for backwards compatibility */
+        size_t fsmblks;  /* space available in freed fastbin blocks */
+        size_t uordblks; /* total allocated space */
+        size_t fordblks; /* total free space */
+        size_t keepcost; /* top-most, releasable (via malloc_trim) space */
+};
+
+static inline struct mallinfo2 mallinfo2(void) {
+DISABLE_WARNING_DEPRECATED_DECLARATIONS
+        struct mallinfo m = mallinfo();
+REENABLE_WARNING
+
+        return (struct mallinfo2) {
+                .arena = m.arena,
+                .ordblks = m.ordblks,
+                .smblks = m.smblks,
+                .hblks = m.hblks,
+                .hblkhd = m.hblkhd,
+                .usmblks = 0,
+                .fsmblks = m.fsmblks,
+                .uordblks = m.uordblks,
+                .fordblks = m.fordblks,
+                .keepcost = m.keepcost,
+        };
+}
+#endif

src/basic/mallinfo-util.h

@@ -1,24 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-#pragma once
-
-#include <malloc.h>
-
-#if HAVE_MALLINFO2
-#  define HAVE_GENERIC_MALLINFO 1
-typedef struct mallinfo2 generic_mallinfo;
-static inline generic_mallinfo generic_mallinfo_get(void) {
-        return mallinfo2();
-}
-#elif HAVE_MALLINFO
-#  define HAVE_GENERIC_MALLINFO 1
-typedef struct mallinfo generic_mallinfo;
-static inline generic_mallinfo generic_mallinfo_get(void) {
-        /* glibc has deprecated mallinfo(), let's suppress the deprecation warning if mallinfo2() doesn't
-         * exist yet. */
-DISABLE_WARNING_DEPRECATED_DECLARATIONS
-        return mallinfo();
-REENABLE_WARNING
-}
-#else
-#  define HAVE_GENERIC_MALLINFO 0
-#endif