new Cog VMs available

CogVM binaries as per VMMaker.oscog-eem.261/r2677.

Move determination of the ammount of headroom to the platform in

osCogStackPageHeadroom (in the various sqFooMain.c files). Hence

2k stack pages on Mac and Win32 with 4k pages on linux.

Provide a routine to monitor the ammount of unused headroom, which

requires the stack memory be zeroed before use.  Assume the platform

will provide a -reportheadroom flag for enabling the report.  Provide

primitiveMinimumUnusedHeadroom for in-image access.

Eliot