testbench: recognize faults in KIP and UTCB areas

And don't attempt to map new memory on top; generate segfaults instead.
test: self: test that forkserv signals segv on UTCB/KIP fault

This property is used in subsequent tests.
space: remove space_put_page() calls from sys_unmap()

It's already done by mapdb_unmap_fpage() in mapdb.c:1583.

The calls were replaced with a loop that, when the flush option is set, checks
that the given rights are absent from the current space's page tables.
test: space: align tested memory range correctly in large_flush

That thing should be in an utility function or something.