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/entry.c

@@ -13,6 +13,8 @@
 
 #include <sys/types.h>
 #include <stdio.h>
+
+#include "alloc.h"
 #include "prototypes.h"
 #include "defines.h"
 #include <pwd.h>