add os::dll_load to unified logging os category

Currently we miss the os::dll_load calls from UL, however loading (attempts) of native shared libs are an important information so I think
adding it would be beneficial.
Example output from a prototype :

./images/jdk/bin/java -Xlog:os Test
[0.005s][info][os] Use of CLOCK_MONOTONIC is supported
[0.005s][info][os] Use of pthread_condattr_setclock is supported
[0.005s][info][os] Relative timed-wait using pthread_cond_timedwait is associated with CLOCK_MONOTONIC
[0.005s][info][os] HotSpot is running with glibc 2.11.3, NPTL 2.11.3
[0.005s][info][os] SafePoint Polling address, bad (protected) page:0x00007fc94fc76000, good (unprotected) page:0x00007fc94fc77000
[0.009s][info][os] attempting shared library load of /mydir/images/jdk/lib/libzip.so
[0.011s][info][os] attempting shared library load of /mydir/images/jdk/lib/libjimage.so
[0.013s][info][os] attempting shared library load of /mydir/images/jdk/lib/libjava.base-coop.so
[0.013s][info][os] attempting shared library load of /mydir/images/jdk/lib/libjava.logging-coop.so
[0.013s][info][os] attempting shared library load of /mydir/images/jdk/lib/libjdk.compiler-coop.so
[0.013s][info][os] attempting shared library load of /mydir/images/jdk/lib/libjdk.scripting.nashorn-coop.so
[0.013s][info][os] attempting shared library load of /mydir/images/jdk/lib/libjdk.internal.vm.ci-coop.so
[0.013s][info][os] attempting shared library load of /mydir/images/jdk/lib/libjdk.internal.vm.compiler-coop.so
[0.114s][info][os] attempting shared library load of /mydir/images/jdk/lib/libnio.so
I am a test