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

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

function                                             old     new   delta
bc_parse_name                                        416       -    -416
zbc_parse_name                                         -     414    +414
bc_parse_parse                                       305       -    -305
zbc_parse_parse                                        -     311    +311
bc_program_read                                      271       -    -271
zbc_program_read                                       -     261    +261
bc_parse_text                                        129       -    -129
zbc_parse_text                                         -     122    +122
dc_parse_parse                                        26       -     -26
zdc_parse_parse                                        -      23     +23
bc_program_index                                      66      64      -2
bc_vm_process                                         92      74     -18
bc_program_execStr                                   460     429     -31
bc_parse_expr_empty_ok                              1939    1882     -57
------------------------------------------------------------------------------
(add/remove: 5/5 grow/shrink: 0/4 up/down: 1131/-1255)       Total: -124 bytes
   text	   data	    bss	    dec	    hex	filename
 981058	    485	   7296	 988839	  f16a7	busybox_old
 980934	    485	   7296	 988715	  f162b	busybox_unstripped

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