[SV 64124] Avoid stack overflows for large command lines

Modify areas dealing with large command lines to use the heap rather
than relying on alloca / stack space.

* src/main.c (main): Allocate potentially large buffers with xmalloc.
(decode_env_switches): Ditto.
* src/function.c (func_error): Replace alloca with xmalloc/free.
* tests/scripts/features/expand: Add a newline for readable diffs.