bc: simplify ^C handling

bc: simplify ^C handling

It's still buggy: if SIGINT would interrupt e.g. output write(),
the result would not be as intended:

>>> while ( 1 ) print 1
...
11111111111111111111111111111111111111111111111111111111111111111111\
11111111111111111111111111111111111111111111111111111111111111111111\
^Cready for more input

interrupt (type "quit" to exit)
>>> bc: output error

function                                             old     new   delta
dc_parse_parse                                        58      53      -5
bc_parse_parse                                       476     471      -5
bc_num_d                                             609     604      -5
bc_num_cmp                                           299     294      -5
bc_num_compare                                        90      84      -6
bc_parse_expr                                       2222    2215      -7
bc_num_subArrays                                      82      74      -8
dc_main                                               72      62     -10
bc_main                                               72      62     -10
bc_program_exec                                     4413    4401     -12
bc_num_a                                             491     474     -17
bc_read_line                                         364     344     -20
bc_num_p                                             540     518     -22
bc_program_reset                                     192     168     -24
bc_num_k                                            1031    1005     -26
bc_vm_run                                           2382    2317     -65
bc_vm_sig                                             97       -     -97
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 0/16 up/down: 0/-344)          Total: -344 bytes
   text	   data	    bss	    dec	    hex	filename
 989372	    485	   7296	 997153	  f3721	busybox_old
 989028	    485	   7296	 996809	  f35c9	busybox_unstripped

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