libmisc: Move xmalloc.c to alloc.c

We'll expand the contents in a following commit, so let's move the file
to a more generic name, have a dedicated header, and update includes.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

Use the new header for xstrdup()

Signed-off-by: Alejandro Colomar <alx@kernel.org>

libmisc/env.c

@@ -15,6 +15,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+
+#include "alloc.h"
 #include "prototypes.h"
 #include "defines.h"
 #include "shadowlog.h"