cd built_in and PWD env var

[jovanbulck]

cd also updates the PWD env var, but the supplied dir can also be relative (. or ..) and should then be resolved in the PWD var. Here's the code:

        case CD: ; // (hackhackhack to allow declarions inside a switch)
            char *dir;
            if (comd->length == 1)
                dir = getenv("HOME");
            else {
                CHK_ARGC("cd",1);
                dir = comd->cmd[1];
            }
            CHK_ERR(chdir(dir), "cd");
            setenv("PWD", dir, 1);   // BUG: dir can be relative and should then be resolved
            return EXIT_SUCCESS;
            break;

[jovanbulck]

See issue #51 for further discussion