(dll_crt0_1): Eliminate user_data initialization.
(dll_crt0): Set up impure_ptr_ptr for older executables.
(cygwin_dll_init): Eliminate user_data initializations.
(__api_fatal): Don't check for user_data initialization.
* dll_init.cc (struct dll): Store entire contents of per_process rather than
just a pointer.
(add): Ditto.
(initOneDll): Don't check for user_data initialization.
(DllList::recordDll): Store contents of per_process argument.
(DllList::detachDll): Pass address of per_process field.
(DllList::initAll): Ditto.
(DllList::doGlobalDestructorsOfDlls): Ditto.
(DllListIterator::operator *): Ditto.
(dll_dllcrt0): Default to __cygwin_user_data if arg is NULL.
* include/sys/cygwin.h: Reorganize per_process to eliminate obsolete fields and
accomodate new way of initializing.
* lib/_cygwin_crt0_common: Initialize _impure_ptr from
__cygwin_user_data.impure_ptr.
