This tries to add information about when each option was added. It goes
back to version 183.
The version info is included from a separate file to allow generating it,
which would allow more control on the formatting of the final output.
sd_bus_get_fd() and related calls are useful for integrating a bus
connection into arbitrary event loops. But sd_bus_set_fd() is quite a
different beast, it's for using D-Bus over pre-initialized sockets or
pairs of fifos or stuff, i.e. very advanced stuff.
Let's split this man page in two, in order not to confuse things
needlessly.
And while we are at it, let's slightly extend the documentation.
