API to set listen() backlog size

[witaly-iwanow]

It's currently hard-coded to 5 which is not a lot imo, it would be nice to have one more API define like CPPHTTPLIB_LISTEN_BACKLOG which you can use to increase the number. I'd create a PR, but I guess I'll spend much more time forking and such than you just adding it to main/master whenever possible. TY

[witaly-iwanow]

The reason I'd like to have that is because I see a lot of SSL errors when load testing with ab -c 6, and all the errors go away with -c 5 or when I bump the backlog size manually in httplib.h. I see this behavior on macOS (M1 Air), while on Ubuntu it has no problems whatsoever even with -c 100 - I suspect the backlog parameter might actually be ignored on Linux

[yhirose]

@witaly-iwanow, I added it!