Allocate environment block before forking.

Allocate environment block before forking.

While 'child_setup' carefully avoids calls to async-signal-unsafe
functions like 'malloc', it seems simpler and less brittle to use
normal allocation outside the critical section between 'fork' and
'exec'.

* src/callproc.c (make_environment_block): New function to create the
environment block for subprocesses.  Code largely extracted from
'child_setup' and adapted to use 'xmalloc' instead of 'alloca'.
(child_setup): Remove environment block allocation in favor of
passing the environment block as command-line argument.
(call_process): Adapt to new calling convention.

* src/process.c (create_process): Adapt to new calling convention.