input/vcd: use common helpers for text lines/words splitting

Use recently added strutil helpers which isolate text lines in receive
buffers and which locate whitespace separated words on text lines. This
simplifies the processing of the data section of VCD input files.

It's essential to _not_ use these helpers when the receive buffer must
not get manipulated, like in file format matches, or when control data
sections span multiple text lines. Update comments to raise awareness.