source/base: Make it possible to get the prepare callback

source/base: Make it possible to get the underlying sd_event_source

source/base: Only disable the event if it exists

This shouldn't happen in practice but will be needed to supporting
moving.

source/base: Take an event directly instead of sdevent

Also expose a mechanism for retrieving our internal event.

source/base: Constify

event: Add trivial functions

code-coverage: Make lcov options configurable

clock: Add class for future use

event: Allow extracting the event and sdevent impl

event: Constify operations

code-coverage: Don't use full paths in HTML output

event: Constify sdevent

sdref: Moves should actually invalidate the reference

This also makes it possible to use in boolean expressions to check
validity.

sdref: Fix styling

sdref: Constify sdevent

sdevent: Cleanup spacing

clang-format: Always break template declarations

test/event: Fix extraneous Event creation

source/base: Add the prepare callback functionality

source/base: Expose trivial built in methods

configure: Make examples optional

configure: Fix help strings

Large refactoring

code-coverage: Ignore system headers and sd_event impl

makefile: Clean up compiler flags