Before this fix, starting the server with
  host_cache_size = N
in the my.cnf file had no effect.
The internal host cache size was always 128.

The root cause is in hostname_cache_init(),
which is using 'HOST_CACHE_SIZE' (a #define to 128)
instead of 'host_cache_size' (the C global variable that corresponds to the
SQL system value named host_cache_size).

Overall, the initialization of the host cache is not satisfactory, and needs
to be cleaned up.

In particular, the fact that there are distinct:
- data path (mysqld.cc setting the host_cache_size global variable, 
  to be used by hostname.cc)
- control path (mysqld.cc calling hostname_cache_init with no parameters)
can only be a source of confusion.

This fix:
- moves the global variable host_cache_size to mysqld, where it belongs,
  because this is a server global variable
- changes the variable type from ulong to uint, given that the size really
  is an unsigned int, per hash_filo::m_size
- changes hostname_cache_init() to take explicitely a size,
  for consistency with hostname_cache_resize(),
  and for a cleaner interface.