net: split illumos out of solaris on TCP keep-alive mechanism

[panjf2000]

Oracle Solaris claimed it supported the TCP-Alives mechanism, but TCP_KEEPIDLE, TCP_KEEPINTVL, and TCP_KEEPCNT were not available on Solaris until the latest version 11.4. If we want to achieve the full TCP-Alives mechanism on solaris (<11.4) that already exists on other platforms, we can only do it by simulating it via TCP_KEEPALIVE_THRESHOLD + TCP_KEEPALIVE_ABORT_THRESHOLD.

Meanwhile, illumos has supported TCP_KEEPIDLE, TCP_KEEPINTVL, and TCP_KEEPCNT a year after it was forked from OpenSolaris (2011). And since GOOS=illumos implies the solaris build tag in Go, illumos and solaris share the same TCP keep-alive implementation, Go programs running on illumos won't be able to benefit from the full TCP keep-alive feature implemented by CL 542275. Besides, issues like #9614 and #64251 are even stirring things up. Therefore, we should split illumos out of solaris on TCP keep-alive.

Follows up #65809

References:

• TCP on Solaris 11.3
• TCP on Solaris 11.4
• TCP on illumos
• Add support for socket options TCP_KEEPCNT, TCP_KEEPIDLE, TCP_KEEPINTVL
• net: setKeepAlivePeriod may be wrong on Solaris
• net: getting setsockopt: invalid argument when setting TCP keep-alive on Solaris
• MinimumRequirements for solaris

[gopherbot]

Change https://go.dev/cl/565335 mentions this issue: net: split the illumos out of solaris on TCP keep-alive mechanism

[gopherbot]

Change https://go.dev/cl/575015 mentions this issue: net: split illumos out of solaris on TCP keep-alive mechanism