Right now, the pkg-config checks for the system version of
libdhash are forcibly disabled, requiring the SSSD to build it
from its own tree. In the future, when we split the libraries off
from the SSSD, it will be easy to switch this check to the
external library.

Also pass a flag to the delete callback to tell it if this is a normal
entry removal or we are cleaning up the tbale definitively.

Apply suggested fixes by Simo after code review
* return statements no longer use () unless it's an expression
* remove all use of assert() in library
* use bool,true,false instead of int,TRUE,FALSE
* add check for NULL hash table in public entry points
* example code in header file now a seperate file

* assure consistent use of unsigned long data type
* add more debugging support
* break out generation of integer key into convert_key() function

* table parameters now tunable rather than hardcoded
* table can now accept custom alloc()/free() functions
* add function create_table_ex() to pass extra table parameters
* remove MUL(), DIV(), MOD() macros
* hash statistics now separate struct which can be queried
* test program now accepts tuning parameters, iteration count; has better error checking and reporting

fix min/max load factor comman line args in test program