core/service: tweak messages on operation timeouts
core/manager: print info about interesting signals

Information about signals which are not routinely received by systemd
are printed at info level. This should make it easier to see what is
happening in the system.
core/service: check if mainpid matches only if it is set

https://bugzilla.redhat.com/show_bug.cgi?id=1047304
Move bus path definitions to def.h
Use format patterns for usec_t, pid_t, nsec_t, usec_t

It is nicer to predefine patterns using configure time check instead of
using casts everywhere.

Since we do not need to use any flags, include "%" in the format instead
of excluding it like PRI* macros.
bus: break reference cycle between bus and messages

Because messages in the read and write queues hold a reference on the
bus, and the bus holds a reference on each of them, we would never
free the bus if the read or write queues were not empty. Explicitly
substract the number of messages in those queue from the bus reference
count when deciding whether to free or not.

A simple test which creates and unrefs simple objects is added.
rtnl: fix memory corruptions after realloc

struct sd_rtnl_message would keep two additional pointers into the hdr
field. Every time hdr was realloced, those pointers should be adjusted,
but weren't. It seems less error-prone to keep offsets instead.
build-sys: make valgrind-tests target output nicer
build-sys: add --disable-dbus autoconf option
Introduce cleanup functions for cap_free

Unfortunately a different cleanup function is necessary per type,
because cap_t** and char** are incompatible with void**.
bus: fix memleak in sd_bus_creds_new_from_pid
core: fix gcc unused variable warning
networkd: do not deference null pointer in cleanup
networkd: fix memory leak in error path