API: test "correct" capitalization of key header

Test what we document, not what the code checks for.
ship notify tool
read ALLOW-AXFR-FROM from the backend with the metadata
Remove leftover function prototype

No function body is to be seen anywhere.
Remove unused define
Fail if we can't find libopendbx
Rename variable to be consistent with the rest
Check for libopendbx with pkg-config
Merge pull request #1869 from zeha/cleanup

Misc. cleanup
Merge pull request #1858 from mind04/notify

ship notify tool
Merge pull request #1845 from zeha/api-test-header-name

API: test "correct" capitalization of key header
Merge pull request #1870 from rubenk/opendbx-autoconf

Autoconf cleanup for opendbx backend
Merge pull request #1862 from mind04/allow-axfr

read ALLOW-AXFR-FROM from the backend with the metadata