internal: use OS defaults for TCP keepalive params only on unix

[easwars]

In #6834, we made a change to ensure that OS defaults for TCP keepalive params are used. But, that change failed to compile on windows. This fix addresses it.

RELEASE NOTES: none

[codecov]

Codecov Report

Merging #6841 (6c63504) into master (477bd62) will decrease coverage by 0.11%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6841      +/-   ##
==========================================
- Coverage   83.65%   83.54%   -0.11%     
==========================================
  Files         286      286              
  Lines       30801    30801              
==========================================
- Hits        25766    25734      -32     
- Misses       3980     4002      +22     
- Partials     1055     1065      +10     

Files	Coverage Δ
internal/tcp_keepalive_unix.go	100.00% <100.00%> (ø)

... and 17 files with indirect coverage changes

[dfawley]

This does seem to work for me. Without any build constraint:

$ GOOS=illumos go build ./internal
$ GOOS=android go build ./internal
$ GOOS=ios go build ./internal
$ GOOS=windows go build ./internal
# google.golang.org/grpc/internal
internal/tcp_keepalive.go:48:10: undefined: unix.SetsockoptInt
internal/tcp_keepalive.go:48:38: undefined: unix.SOL_SOCKET
internal/tcp_keepalive.go:48:55: undefined: unix.SO_KEEPALIVE

And with the build constraint, the windows build passes.

[easwars]

I didn't realize we could build for any OS with GOOS. Thanks.

[andyliuliming]

looks like this PR will broke the referencing grpc from bazel.

[dfawley]

@andyliuliming can you file an issue and let us know what's wrong so we can try to fix it?

[andyliuliming]

@andyliuliming can you file an issue and let us know what's wrong so we can try to fix it?

thanks, opened one issue:
#6929
trying to create a mock repo with the minimal repro.