Don't call "ulimit" on cygwin: On Cygwin the stack size cannot be modified, and the reported stacksize is much lower than the real one.
Extend the public and private stub tables with dummy NULL entries, up to the size of the Tcl 8.6 stub tables. 
This makes it easier to debug extensions which use Tcl 8.5/8.6 features but (erroneously) are attempted to be loaded in Tcl 8.4.
Extend the public stub table with dummy NULL entries, up to the size of the Tcl 8.6 stub tables. 
This makes it easier to debug extensions which use Tcl 8.6 features but (erroneously) are attempted to be loaded in Tcl 8.5.