output/ascii: data type nits, rephrase sample bit access

output/ascii: data type nits, rephrase sample bit access

Use size types for counters, unsigned for bit manipulation. Trigger
position needs to remain a signed int (must be possible to go negative
for "not here", and strictly remains within the output text line length,
so should be good).

Rephrase the nested loop during bit extraction from logic packets, and
how a channel's value at a given sample number gets accessed. Eliminate
redundancy in that spot, to improve readability and simplify maintenance.