bug#44173: 28.0.50; gdb-mi mangles strings with octal escapes

[Mattias Engdegård]

When GDB sends a string containing octally-escaped characters, it will be 
something like

 "abc\377def"

which is first massaged into JSON and then parsed as such, but since JSON 
doesn't recognise octal escapes, the result is

 "abc377def"

which is wrong. This assumes gdb-mi-decode-strings is nil, which it is by 
default; otherwise the octal escapes are decoded by a fragile preprocessing 
stage (gdb-mi-decode) which itself has known problems as noted in a comment.

Frankly, this business with going via JSON after several ad-hoc text-based 
transforms isn't very principled.

While the bug could be 'solved' by adding yet another regexp hack to 
gdb-mi-decode or gdb-mi-jsonify-buffer, I suggest we write a GDB/MI parser in 
Lisp directly, ditching the gdb-mi-decode preprocessing and JSON form entirely, 
solving this and related bugs once and for all. Many transforms can then be 
done on the S-expression result after parsing, which should be more efficient 
and less error-prone.