prog: more readable serialization for strings

[dvyukov]

Dominique Martinet reported at:
https://groups.google.com/d/msg/syzkaller-bugs/axCkZsIU5KI/C4NDCPRtAwAJ

the C reproducer (
https://syzkaller.appspot.com/x/repro.c?x=1701f831400000 ) has a lot of
"readable" letters spelled out as "\x63..." or chars as 0x3d - it's fine
for generated code and that might be easier for the intermediate
representation syzkaller works with, but do you know something handy
that would help convert these to readable strings?
e.g. "\x63\x61\x63\x68\x65\x3d\xc0\x6d\x61\x70" could be written
"cache=\xc0map", and 0x3d as '=' (hm I guess the later would not always
make sense to convert so probably best left as is, but it gets annoying
pretty fast with longer strings)

We now have readable format for strings:
dvyukov@06456ca
but it is used only if the whole string is readable. We could use it more widely.

And need to ensure that pkg/csource also generates readable strings.

[martinetd]

(I'll blindly trust you on this), thanks!

[dvyukov]

I re-generated the original program with this change and it contained the desired "cache=\xc0map". You are welcome.