Implemented a default password reading callback for yaSSL
using mysql client's get_tty_password().
It does that by:
- Implementing an extended version of get_tty_password()
called get_tty_password_ext() that takes a strdup function 
pointer.
- Added a header file to keep the definition of the new 
function out of ABI
- Adds client/get_password.c to the yassl lib and uses the 
C preprocessor to rename get_tty_passord() and 
get_tty_password_ext() to names prefixed
with yassl and internal to the yassl library. We need to keep
the two versions of get_tty_password_ext() to keep yassl
self sufficient (with no cross references to other libraries).

Since the password can't be read from a file, only a manual
test performed and no automatic test case added.