tree-wide: don't assume CLOCK_BOOTIME is generally available

tree-wide: don't assume CLOCK_BOOTIME is generally available

Before we invoke now(CLOCK_BOOTTIME), let's make sure we actually have that
clock, since now() will otherwise hit an assert.

Specifically, let's refuse CLOCK_BOOTTIME early in sd-event if the kernel
doesn't actually support it.

This is a follow-up for #3037, and specifically:

https://github.com/systemd/systemd/pull/3037#issuecomment-210199167