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/wchan.h

@@ -5,8 +5,6 @@
 
 EXTERN_C_BEGIN
 
-typedef void (*message_fn)(const char *restrict, ...) __attribute__((format(printf,1,2)));
-
 extern const char * lookup_wchan(unsigned KLONG address, unsigned pid);
 extern int   open_psdb(const char *restrict override);
 extern int   open_psdb_message(const char *restrict override, message_fn message);