stacked-environ:  Remove stacking code
libc:  Ignore environ in vfork()

Calling code no longer expects separate parent and child environments to
exist under vfork().
stacked-environ:  Remove environ_store::pop()
stacked-environ:  Remove environ chaining
readonly-environ:  Remove stacking code
relix:  Remove environ_stack.h
Merge branch 'environ_stack'

The _{push,pop}_environ() functions are no longer called, so all of this
code is dead or useless.
stacked-environ:  Sort the environment variables

We keep the environment variables sorted on insert, but we were assuming
they'd come in sorted, which isn't necessarily true at all.