shell/read: fix "'read' without parameters" bash compat thingy

shell/read: fix "'read' without parameters" bash compat thingy

previous change:

function                                             old     new   delta
builtin_read                                          82    1074    +992
popstring                                            134     140      +6
readcmd                                             1034     148    -886
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 998/-886)          Total: 112 bytes

this change:

builtin_read                                        1074    1096     +22
static.arg_REPLY                                       8       -      -8
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 1/0 up/down: 22/-8)              Total: 14 bytes

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