bc: fold single-callsite functions into callers, commonalize code

bc: fold single-callsite functions into callers, commonalize code

function                                             old     new   delta
zbc_vm_process                                       699     745     +46
zbc_parse_stmt_possibly_auto                        1689    1682      -7
bc_parse_reset                                       113       -    -113
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(add/remove: 0/1 grow/shrink: 1/1 up/down: 46/-120)           Total: -74 bytes

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