Several functions including wgetopt and execve operate on null-terminated
arrays of nul-terminated pointers: a list of pointers to C strings where
the last pointer is null. Prior to this change, each process_t stored its
argv in such an array. This had two problems:
1. It was awkward to work with this type, instead of using std::vector,
etc.
2. The process's arguments would be rearranged by builtins which is
surprising
Our null terminated arrays were built around a fancy type that would copy
input strings and also generate an array of pointers to them, in one big
allocation.
Switch to a new model where we construct an array of pointers over
existing strings. So you can supply a `vector<string>` and now
`null_terminated_array_t` will just make a list of pointers to them. Now
processes can just store their argv in a familiar wcstring_list_t.
Prior to this change, builtins would take their arguments as `wchar_t **`.
This implies that the order of the arguments may be changed (which is
true, `wgetopter` does so) but also that the strings themselves may be
changed, which no builtin should do.
Switch them all to take `const wchar_t **` instead: now the arguments may
be rearranged but their contents may no longer be modified.
