input/csv: improve "channel name from header line" logic

input/csv: improve "channel name from header line" logic

Improve the code paths which determine logic channels' names from an
optional CSV file header line. Strip optional quotes from the column's
input text (re-use a SCPI helper routine for that). Also use the channel
name for multi-bit fields, append [0] etc suffixes in that case. Comment
on the manipulation of input data, which is acceptable since that very
data won't get processed another time in another code path.