net/ipforward: it should have a ipforward setting based interface

[masc2008]

Summary

It's like option "net.ipv4.conf.eth0.forwarding" in linux.
It's a useful setting if we need control ip forward by network interface level. Such like a LTE modem, a LTE modem usually has 3 net interfaces, while only one can be used data communication. It allows us configure the net device usage more flexible.

Impact

• Nuttx support ipforward control, while it is a system level control for all network interfaces. This commit is to improve configure flexibility.
• This need user application to enable a network interface forward capability. we can set ipfoward flag is enabled by default if we need to keep backward compatibility. it's a reasonable to choose ipforward flag disabled by default.
• Impact on build (will build process change)? NO
• Impact on hardware (will arch(s) / board(s) / driver(s) change)? NO
• Impact on documentation (is update required / provided)? YES (If enable CONFIG_NET_IPFORWARD, need enable at least one net interface as "ipforward").
• Impact on security (any sort of implications)? YES (This helps improve the security, since we choose specific net interface that can do ipforward, before the fwdev is choose by default.).
• Impact on compatibility (backward/forward/interoperability)? YES (as mentioned above, it do changes some behavior, while it's something that deserve).

Testing

I confirm that changes are verified on local setup and works as intended:

• Build Host(s): OS (Linux,BSD,macOS,Windows,..), CPU(Intel,AMD,ARM), compiler(GCC,CLANG,version), etc.
• Target(s): ARM, BES chips.

Testing logs before change:

any device can be chosen as ipforward net interface.

Testing logs after change:

with this commit, only network interface with flag "IFF_FORWARD" can forward ip packet.

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation (git commit title and message, coding standard, etc).
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

[xiaoxiang781216]

where you call this function, please follow how IFUP get changed from userspace

[masc2008]

could you give me more further info about this "how IFUP" get changed from userspace. is there any commit id or related topic for it?

[masc2008]

if you already have this one, I think this PR can be abandoned, thanks : )

[linguini1]

Please fill out the PR description according to the template + contributing guidelines.

[xiaoxiang781216]

@masc2008 please review #18260 (review), which implement the similiar and more functionality.