bc: rename zbc_parse_text() to ..._init(), reuse existing code for library parsing

bc: rename zbc_parse_text() to ..._init(), reuse existing code for library parsing

function                                             old     new   delta
zbc_parse_text_init                                    -     123    +123
bc_vm_run                                            423     398     -25
zbc_parse_text                                       123       -    -123
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 0/1 up/down: 123/-148)          Total: -25 bytes
   text	   data	    bss	    dec	    hex	filename
 980154	    485	   7296	 987935	  f131f	busybox_old
 980129	    485	   7296	 987910	  f1306	busybox_unstripped

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