bc: parse file arguments piecemeal (do not read entire file)

bc: parse file arguments piecemeal (do not read entire file)

function                                             old     new   delta
bc_read_line                                         336     406     +70
zbc_vm_execute_FILE                                    -      67     +67
zbc_lex_next                                        2309    2318      +9
zbc_program_exec                                    4002    4008      +6
bc_program_index                                      66      64      -2
bc_vm_run                                            139     124     -15
zbc_vm_file                                          208      32    -176
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 3/3 up/down: 152/-193)          Total: -41 bytes
   text	   data	    bss	    dec	    hex	filename
 981736	    485	   7296	 989517	  f194d	busybox_old
 981667	    485	   7296	 989448	  f1908	busybox_unstripped

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