add function --shadow-builtin flag

It's currently too easy for someone to bork their shell by doing something
like `function test; return 0; end`. That's obviously a silly, contrived,
example but the point is that novice users who learn about functions are
prone to do something like that without realizing it will bork the shell. Even
expert users who know about the `test` builtin might forget that, say, `pwd`
is a builtin.

This change adds a `--shadow-builtin` flag that must be specified to
indicate you know what you're doing.

Fixes #3000

src/exec.cpp

@@ -636,7 +636,7 @@ void exec_job(parser_t &parser, job_t *j) {
                 wcstring def;
                 bool function_exists = function_get_definition(func_name, &def);
 
-                bool shadows = function_get_shadows(func_name);
+                bool shadow_scope = function_get_shadow_scope(func_name);
                 const std::map<wcstring, env_var_t> inherit_vars =
                     function_get_inherit_vars(func_name);
 
@@ -646,7 +646,7 @@ void exec_job(parser_t &parser, job_t *j) {
                     debug(0, _(L"Unknown function '%ls'"), p->argv0());
                     break;
                 }
-                function_block_t *newv = new function_block_t(p, func_name, shadows);
+                function_block_t *newv = new function_block_t(p, func_name, shadow_scope);
                 parser.push_block(newv);
 
                 // Setting variables might trigger an event handler, hence we need to unblock