library: expanded/generalized memory allocation provisions

A callback provision in the form of xalloc_err_handler
(of type message_fn) was added to the alloc module.

This change allowed a program like top, who alters the
termios structure, to override the default fprint(stderr...)
behavior in the event of an error.

The new function xstrdup was also added for symmetry.

proc/procps.h

@@ -109,4 +109,7 @@
 #define HIDDEN_ALIAS(x) extern __typeof(x) x##_direct __attribute__((alias(#x)))
 #endif
 
+
+typedef void (*message_fn)(const char *restrict, ...) __attribute__((format(printf,1,2)));
+
 #endif