Update security polling status for Debian and derivatives

Current versions.
Replace MYSQL_inc with MYSQL_CFLAGS

Part of #1748
Replace MYSQL_lib with MYSQL_LIBS

Part of #1748
Allow overriding of LDAP_LIBS

Part of #1826
Add some pointers to virtualenv to the docs
Merge pull request #1920 from pieterlexis/doc-mkdocs

Add some pointers to virtualenv to the docs
Always assume HAVE_IPV6

With this Solaris 2.6 and 7 is strictly unsupported, as we remove the
configure.ac special casing for them.
Sync schema with mydns-ng
Support mydns-ng style Y/N active field.
improve some wording, remove unneeded \_ escapes
Merge pull request #1636 from zeha/remove-have-ipv6

Always assume HAVE_IPV6
Restore 'aux'
log client IP for immediate SERVFAIL exceptions
improve cache hitrate
Port port
remove some dead code
enable callback based metrics to statbas, and add 5 such metrics: uptime, sys-msec, user-msec, key-cache-size, meta-cache-size, signature-cache-size
fix up statbag for carbon protocol and function pointers
Merge pull request #1886 from zeha/secpoll-debian

Update security polling status for Debian
docs: recursor/running: cleanup

Mark all config vars bold, all commands/parameters preformatted,
remove double escaping.