RFR: 8252657: JVMTI agent is not unloaded when Agent_OnAttach is failed

[Yasumasa Suenaga]

If `Agent_OnAttach()` in JVMTI agent which is attempted to load via JVMTI.agent_load dcmd is failed, it would not be
unloaded.   We've [discussed it on
serviceability-dev](https://mail.openjdk.java.net/pipermail/serviceability-dev/2020-September/032839.html). This PR is
a continuation of that.

This PR also includes to call `Agent_OnUnload()` when `Agent_OnAttach()` failed.

How to reproduce:

1. Build JVMTI agent for test
$ git clone https://github.com/YaSuenag/jvmti-examples.git
$ cd jvmti-examples/helloworld/out/build
$ cmake ../..

2. Run JShell

3. Load JVMTI agent via `jcmd <PID> JVMTI.agent_load` with "error" ("error" means `Agent_OnAttach()` returns JNI_ERR)
$ jcmd
89456 jdk.jshell.execution.RemoteExecutionControl 45651
89547 sun.tools.jcmd.JCmd
89436 jdk.jshell/jdk.internal.jshell.tool.JShellToolProvider
$ jcmd 89436 JVMTI.agent_load `pwd`/libhelloworld.so error
89436:
return code: -1

4. Check loaded libraries via `jcmd <PID> VM.dynlibs`
$ jcmd 89436 VM.dynlibs | grep libhelloworld
7f2f8b06b000-7f2f8b06c000 r--p 00000000 fd:00 11818202
/home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so 7f2f8b06c000-7f2f8b06d000 r-xp 00001000
fd:00 11818202 /home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so 7f2f8b06d000-7f2f8b06e000
r--p 00002000 fd:00 11818202 /home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so
7f2f8b06e000-7f2f8b06f000 r--p 00002000 fd:00 11818202
/home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so 7f2f8b06f000-7f2f8b070000 rw-p 00003000
fd:00 11818202 /home/ysuenaga/github/jvmti-examples/helloworld/out/build/libhelloworld.so

-------------

Commit messages:
 - JVMTI agent is not unloaded when Agent_OnAttach is failed

Changes: https://git.openjdk.java.net/jdk/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=19&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252657
  Stats: 44 lines in 4 files changed: 32 ins; 6 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/19/head:pull/19

PR: https://git.openjdk.java.net/jdk/pull/19