Add cacert option for curl plugin

    add cacert option for Curl plugin. Allows to set cacert for curl lib
    Added documentation line into doc/plugins.rst with explanation for cacert option

This is semi-standard and doesn't require the util/Compiler.h header.

Throwing std::runtime_error was slightly wrong.

Commit 79b23663 added the field `skip`
to support unaligned reads, but set the `offset` field to a wrong
value.  This resulted in miscalculation of `remaining`, causing
an assertion failure.

The fix is to assign `offset` the correct value, but consider the
`skip` value in the assertion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1067

After a Seek() to an odd offset, some data needs to be skipped from
the start of the block, and reading right into the given buffer
doesn't work.

Conductor was incorrectly saved to Performer tag in MPD database

... and make TimerEvent a type alias for FineTimerEvent (i.e. swap
names).

Preparing for a variant of TimerEvent with coarse 1-second
granularity, but cheaper (with a timer wheel).

zeroconf/ZeroconfBonjour: Fix compile errors resulting from "refactor to SocketEvent" and reenable bonjour for darwin build

Fixes Windows compatibility.