hush: handle LINENO the same way as RANDOM: variable is "ephemeral"

hush: handle LINENO the same way as RANDOM: variable is "ephemeral"

"env - hush" invocation (that is, with empty environment)
should not show LINENO in "set" output.

function                                             old     new   delta
get_local_var_value                                  263     294     +31
hush_main                                           1105    1070     -35
handle_changed_special_names                          79      38     -41
run_pipe                                            1834    1765     -69
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/3 up/down: 31/-145)          Total: -114 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>