bc: partially rewrite parser, tests pass, ^C might be broken now

bc: partially rewrite parser, tests pass, ^C might be broken now

The entire control construct (if/while/for/funcdef) or {} block is
"eaten" by the corresponding parsing function, instead of maintaining
special "block flag stack" with magic bits in it, and returning to main
input loop after every inner statement (every input line, essentially).

This required moving line input deep into lexer - now zbc_lex_next()
triggers more reading when needed.

"block flag stack" is gone.

Correctness of ^C handling wasn't checked, might need fixing now.

if/else syntax is changed to match GNU bc: "else" can not be on
the next line (the rationale is that "if (1) 2" statement in interactive
mode should execute and print 2 instead of waiting for possible
"else ..." line).

This change fixes the following examples:

if (1) if (1) 1 else 2 else 3

if (0) 1 else if (1) 2

define w() { auto z; return 1; }

function                                             old     new   delta
zbc_parse_stmt_possibly_auto                           -    2232   +2232
zbc_vm_process                                        89     561    +472
zbc_lex_next                                        1982    2296    +314
bc_vm_init                                           749     757      +8
bc_parse_expr_empty_ok                              2016    2021      +5
bc_num_printNewline                                   54      51      -3
zbc_program_read                                     289     280      -9
bc_parse_free                                         47      38      -9
bc_parse_reset                                       126     113     -13
bc_parse_create                                      108      92     -16
bc_parse_push_block_flag                              47       -     -47
bc_parse_noElse                                       48       -     -48
zbc_parse_text_init                                  113      59     -54
zbc_parse_body                                       121       -    -121
zbc_parse_else                                       125       -    -125
zbc_parse_endBody                                    254       -    -254
bc_vm_run                                            421     134    -287
zbc_parse_auto                                       290       -    -290
zcommon_parse                                        476       -    -476
zbc_parse_stmt                                      1682       7   -1675
------------------------------------------------------------------------------
(add/remove: 1/7 grow/shrink: 4/8 up/down: 3031/-3427)       Total: -396 bytes
   text	   data	    bss	    dec	    hex	filename
 982586	    485	   7296	 990367	  f1c9f	busybox_old
 982138	    485	   7296	 989919	  f1adf	busybox_unstripped

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