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>

src/chage.c

@@ -25,6 +25,8 @@
 #endif				/* USE_PAM */
 #endif				/* ACCT_TOOLS_SETUID */
 #include <pwd.h>
+
+#include "alloc.h"
 #include "prototypes.h"
 #include "defines.h"
 #include "pwio.h"

src/chfn.c

@@ -17,6 +17,8 @@
 #include <stdio.h>
 #include <sys/types.h>
 #include <getopt.h>
+
+#include "alloc.h"
 #include "defines.h"
 #include "getdef.h"
 #include "nscd.h"

src/chsh.c

@@ -16,6 +16,8 @@
 #include <pwd.h>
 #include <stdio.h>
 #include <sys/types.h>
+
+#include "alloc.h"
 #include "defines.h"
 #include "getdef.h"
 #include "nscd.h"