input/trace32_ad: rephrase the header parse logic

input/trace32_ad: rephrase the header parse logic

There were several issues with the previous implementation of the logic
which parses trace32 input files: A string comparison was inverted, and
compared a seven character literal to a five character copy of the input.
One more character was trimmed before CTRL-Z (the CP/M EOF), which often
is SPACE in input files, but might be excessive on other input files.

Replace a DIY character search while we are here. Use symbolic names for
special characters. Factor out a test and memory release, to simplify
error code paths in the remaining logic. Extend comments.