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_stmt                                       1594       -   -1594
zbc_parse_stmt                                         -    1540   +1540
common_parse_expr                                     43       -     -43
zcommon_parse_expr                                     -      40     +40
bc_parse_expr                                         24       -     -24
zbc_parse_expr                                         -      24     +24
zbc_parse_body                                       108     104      -4
bc_program_read                                      277     271      -6
bc_program_execStr                                   468     460      -8
bc_parse_expr_empty_ok                              1955    1939     -16
bc_parse_name                                        451     416     -35
bc_parse_parse                                       476     305    -171
------------------------------------------------------------------------------
(add/remove: 3/3 grow/shrink: 0/6 up/down: 1604/-1901)       Total: -297 bytes
   text	   data	    bss	    dec	    hex	filename
 981355	    485	   7296	 989136	  f17d0	busybox_old
 981058	    485	   7296	 988839	  f16a7	busybox_unstripped

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