Disable the gdb packet run length encoding for 3-symbol repetitions.
While it is technically possible to encode them, they have no advantage
over sending the characters verbatim (the resulting length is the same)
and they result in sending non-printable \x1f character. The protocol
has been designed with the intent of avoiding non-printable characters
and therefore the run length encoding is biased to emit \x20 (a space)
with the minimal intended run length of 4.
While at it, simplify the logic a bit by merging the 'runlen == 1'
clause with 'runlen == 6 || ...', and moving 'runlen == 0' check lower.
The special handling for 'runlen == 2' is added into the existing block
clause for 'runlen == 7'.
Sponsored by: The FreeBSD Foundation