nuttx: add SO_KEEPALIVE and TCP keepalive constants

[no1wudi]

Description

rust-lang/rust#156790

Add SO_KEEPALIVE socket option and TCP_KEEPIDLE, TCP_KEEPINTVL, TCP_KEEPCNT
TCP-level keepalive constants for the NuttX target.

Sources

• SO_KEEPALIVE = 5: https://github.com/apache/nuttx/blob/master/include/sys/socket.h
• TCP_KEEPIDLE, TCP_KEEPINTVL, TCP_KEEPCNT: https://github.com/apache/nuttx/blob/master/include/netinet/tcp.h

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are included
• Tested locally

@rustbot label +stable-nominated

[JohnTitor]

The change looks good. Removed "Fix" issue as you'd also need to update the libc version on r-l/r as well.
The CI failure should be unrelated but I don't have enough time to investigate today. If it continues I'll investigate it tomorrow or so.

View changes since this review

[JohnTitor]

Now it has been fixed on the latest nightly, let's go ahead.