Support multiple cpu-keepalive contexts for single process

Clients that do not provide context information still work as long
as they do not have more than one simulatenously active keepalive
period.

For clients that provide the context information, the cpu keepalive
period ends only when all contexts have been stopped.

If client drops from D-Bus, all contexts are released immediately.

[mce] Support multiple cpu-keepalive contexts for single process
Strip diagnostic messages before writing them to stderr

For example dbus error names contain newline characters. By stripping
the message it still takes just one line in the output.
Add sender name to debug logging from cpu-keepalive method call handlers
Check for NULL strings before strcmp()
Send method reply message even if method call message is malformed

A reply message should always be sent, unless method call message
has no-reply set. And that is handled within cpu_keepalive_reply_bool().
Merge pull request #114 from spiiroin/cpu_keepalive

Support multiple cpu-keepalive contexts from one process