output: fixup trigger marker position in ascii/bits/hex output modules

output: fixup trigger marker position in ascii/bits/hex output modules

Adjust the calculation of the '^' marker's position in T: lines of the
-O ascii/bits/hex output modules such that it matches the sample data
lines' layout. Add comments which discuss the motivation of the marker
position's calculation, which differs among each of those modules.

Strictly speaking -O bits was already correct. But I chose to adjust and
comment the logic such that multiple output modules follow a common
pattern. If performance is an issue, the bits.c change might be worth
reverting.

This commit fixes bug #1238.