6in4: improve HE tunnel update procedure

6in4: improve HE tunnel update procedure

- uclient-fetch timeout bumped from 5s to 15s. If we do not do this
  we get flagged by HE as the update request is expensive and takes
  more than 5s to execute. Currently 5s timeout causes uclient-fetch
  to be killed prematurely as can be seen by the following log:

  10:34:57 user.notice 6in4-henet: update 1/3: timeout
  10:35:07 user.notice 6in4-henet: update 2/3: timeout
  10:35:17 user.notice 6in4-henet: update 3/3: timeout
  10:35:22 user.notice 6in4-henet: update failed

  The above is the worst case, what usually happens is:

  10:53:59 user.notice 6in4-henet: update 1/3: timeout
  10:54:06 user.notice 6in4-henet: update 2/3: abuse
  10:54:06 user.notice 6in4-henet: updated

- We now use an exponential backoff starting from 5 seconds.

- Detect ca-bundle so we don't use --no-check-certificates
  unnecessarily.

- The while loop was changed so we don't retry unnecessarily
  after the final failure.

- Worst-case total time the update operation might take before
  bailing out is:

     (sum(15 + (5 × (2^(x − 1))), 1, 2) + 15) seconds = 1 min

Signed-off-by: Rany Hany <rany_hany@riseup.net>
Link: https://github.com/openwrt/openwrt/pull/22016
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>