Add new functions flag -V/--inherit-variable

--inherit-variable takes a variable name and snapshots its current
value. When the function is executed, it will have a local variable with
this value already defined. Printing the function source will include
synthesized `set -l` lines for the values.

This is primarily useful for functions that are created on the fly, such
as in `psub`.

exec.cpp

@@ -856,6 +856,7 @@ void exec_job(parser_t &parser, job_t *j)
 
                 wcstring_list_t named_arguments = function_get_named_arguments(p->argv0());
                 bool shadows = function_get_shadows(p->argv0());
+                std::map<wcstring,env_var_t> inherit_vars = function_get_inherit_vars(p->argv0());
 
                 signal_block();
 
@@ -868,12 +869,16 @@ void exec_job(parser_t &parser, job_t *j)
                 parser.push_block(newv);
 
                 /*
-                  set_argv might trigger an event
+                  setting variables might trigger an event
                   handler, hence we need to unblock
                   signals.
                 */
                 signal_unblock();
                 parse_util_set_argv(p->get_argv()+1, named_arguments);
+                for (std::map<wcstring,env_var_t>::const_iterator it = inherit_vars.begin(), end = inherit_vars.end(); it != end; ++it)
+                {
+                    env_set(it->first, it->second.missing() ? NULL : it->second.c_str(), ENV_LOCAL | ENV_USER);
+                }
                 signal_block();
 
                 parser.forbid_function(p->argv0());