Make char* parameters const

Make char* parameters const

- Use const in function signatures whenever appropriate, to help prevent
  errant scribbling on users' buffers. smbc_set_credentials() always acted as
  if its formal parameters were const char *, and changing the formal
  declaration to specify that should not cause any change to the ABI. It is
  still allowable to pass a writable buffer to a function which specifies that
  it will not write to the buffer.

  I'm making this change only in master.

Derrell