In elf image activator, read and apply the stack protection mode from
PT_GNU_STACK program header, if present and enabled. Two new sysctls
are provided, kern.elf32.nxstack and kern.elf64.nxstack, that allow to
enable PT_GNU_STACK for ABIs of specified bitsize, if ABI decided to
support shared page.

Inform rtld about access mode of the stack initial mapping by
AT_STACKPROT aux vector.

At the moment, the default is disabled, waiting for the usermode
support bits.