bc: convert more of lexer functions to "z" convention

bc: convert more of lexer functions to "z" convention

function                                             old     new   delta
dc_parse_expr                                        748       -    -748
zdc_parse_expr                                         -     688    +688
bc_parse_endBody                                     338       -    -338
zbc_parse_endBody                                      -     343    +343
bc_parse_auto                                        201       -    -201
zbc_parse_auto                                         -     198    +198
dc_parse_mem                                         115       -    -115
zdc_parse_mem                                          -     108    +108
bc_parse_body                                        116       -    -116
zbc_parse_body                                         -     108    +108
bc_parse_string                                      102       -    -102
zbc_parse_string                                       -     100    +100
dc_parse_register                                     43       -     -43
zdc_parse_register                                     -      40     +40
common_parse_expr                                     40      43      +3
bc_parse_parse                                       321     476    +155
bc_parse_stmt                                       1598    1594      -4
dc_parse_parse                                        44      26     -18
bc_parse_reset                                       153       -    -153
------------------------------------------------------------------------------
(add/remove: 7/8 grow/shrink: 2/2 up/down: 1743/-1838)        Total: -95 bytes
   text	   data	    bss	    dec	    hex	filename
 981450	    485	   7296	 989231	  f182f	busybox_old
 981355	    485	   7296	 989136	  f17d0	busybox_unstripped

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