[Axis2] implemented strdup

[Samisa Abeysinghe]

I wrote our own implementation of strdup. This is because, the default 
strdup that we get with libs are not ANSI C complient.
Please use the AXIS2_STRDUP macro in the Axis2C code when you want to 
duplicate strings.
The resulting pointer has to be freed using AXIS2_FREE
Thanks,
Samisa...