* include/wget/wget.h: Remove allocation function prototypes,
  add function pointer types and 'extern' globals
* libwget/private.h: Use wget_free() instead of free()
* libwget/xalloc.c: Remove function definitions,
  add global function pointer variables
* src/options.c: Add own allocation functions
* unit-tests/test.c: Test that function pointers are overridable