sourcestats: remove tracking of skew change

This is not used since commit 7a6ee1d.
configure: don't remove config files with --help
configure: unify macro naming for available headers
configure: allow building without cmdmon, NTP, refclock support
configure: unify macro naming for optional features
Print enabled/disabled features with version
configure: add --disable-sechash option
nameserv: move fallback DNS_Name2IPAddressAsync() to stubs.c