ifupdown: if $PATH is not set, do not set it to "" in children

ifupdown: if $PATH is not set, do not set it to "" in children

Also random code shrink:

function                                             old     new   delta
set_environ                                          328     330      +2
static.label_buf                                       4       -      -4
startup_PATH                                           4       -      -4
my_environ                                             4       -      -4
ifupdown_main                                       2148    2134     -14
setlocalenv                                          117      90     -27
get_var                                              139     112     -27
------------------------------------------------------------------------------
(add/remove: 0/3 grow/shrink: 1/3 up/down: 2/-80)             Total: -78 bytes

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