print the address of the "non-MAP_STACK stack", so it can be compared
against failure reported in dmesg (hmm, is it time to delete those fault
messages?)

test _exit(2) rather than exit(3) [to keep things simpler], and
print the stack address [for comparison in dmesg post-fault]

Add a test for stack pivots that trigger page faults.

"Regress is always open for commits" @deraadt