doctor: combine git existence and version checks
doctor: combine git origin checks
berkeley-db: --enable-compat185 by default

Our berkeley db shadows the OS's berkeley db 1.x, making it impossible
to build software that only builds on 1.x (e.g. opensmtpd).
The --enable-compat185 switch enables a 1.x compatibility mode, which
shouldn't have any downsides and is the standard in other package
managers. (One wonders why it isn't on by default if so.)