configure: Improve MPTCP kernel detection.

[ossama-othman]

Detect a MPTCP capable kernel through the "enabled" file in
`/proc/sys/net/mptcp' rather than the <linux/mptcp.h> header alone if
the user hasn't chosen a specific MPTCP kernel, i.e. "upstream" or
"multipath-tcp.org". This allows mptcpd to be configured for the
multipath-tcp.org kernel by default if that kernel is at detected at
configure-time rather than always falling back on the upstream
kernel.

Previously, the `configure' script would by default fallback on the
upstream kernel support instead of the multipath-tcp.org kernel when
run on a distro using the latter.

[coveralls]

Pull Request Test Coverage Report for Build 1839053315

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 53.266%

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Totals
Change from base Build 1790578941:	0.0%
Covered Lines:	1060
Relevant Lines:	1990

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💛 - Coveralls

[mjmartineau]

Looks good to me, successful build and test with both 5.16.8 (upstream, Fedora 35 distro kernel) and multipath-tcp.org (4.19.55), they were detected correctly.