awk: fix a case with multiple -f options. simplify -f file reading.

awk: fix a case with multiple -f options. simplify -f file reading.

function                                             old     new   delta
parse_expr                                           833     841      +8
qrealloc                                              33      36      +3
next_input_file                                      203     198      -5
afopen                                                22       -     -22
ftello                                                41       -     -41
ftell                                                 41       -     -41
__GI_ftell                                            41       -     -41
awk_main                                            1002     944     -58
------------------------------------------------------------------------------
(add/remove: 0/4 grow/shrink: 2/2 up/down: 11/-208)          Total: -197 bytes
   text    data     bss     dec     hex filename
 804232     610    6804  811646   c627e busybox_old
 804120     610    6804  811534   c620e busybox_unstripped