RFR: 8265505: findsym does not work on remote debug server [v2]

[Yasumasa Suenaga]

> We can see following error when we run `findsym` on CLHSDB which connects to remote debug server.
> 
> 
> hsdb> verbose true
> hsdb> findsym gHotSpotVMTypes
> 0x00007f913d4a45b0Error: java.lang.NullPointerException: Cannot invoke "sun.jvm.hotspot.debugger.cdbg.CDebugger.loadObjectContainingPC(sun.jvm.hotspot.debugger.Address)" because "cdbg" is null
> java.lang.NullPointerException: Cannot invoke "sun.jvm.hotspot.debugger.cdbg.CDebugger.loadObjectContainingPC(sun.jvm.hotspot.debugger.Address)" because "cdbg" is null
>         at jdk.hotspot.agent/sun.jvm.hotspot.CommandProcessor$7.doit(CommandProcessor.java:618)
>         at jdk.hotspot.agent/sun.jvm.hotspot.CommandProcessor.executeCommand(CommandProcessor.java:2116)
>         at jdk.hotspot.agent/sun.jvm.hotspot.CommandProcessor.executeCommand(CommandProcessor.java:2086)
>         at jdk.hotspot.agent/sun.jvm.hotspot.CommandProcessor.run(CommandProcessor.java:1957)
>         at jdk.hotspot.agent/sun.jvm.hotspot.CLHSDB.run(CLHSDB.java:112)
>         at jdk.hotspot.agent/sun.jvm.hotspot.CLHSDB.main(CLHSDB.java:44)
>         at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.runCLHSDB(SALauncher.java:282)
>         at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.main(SALauncher.java:493)
> 
> 
> The cause of this NPE is that CDebugger is null.  It happens when the debugger is running on debugd server.
> Command line debugger like CLHSDB can delegate the command to debugd, like `pmap` and `pstack`. `findsym` can also use this scheme.
> 
> This PR has passed serviceability/sa tests on Linux x64.

Yasumasa Suenaga has updated the pull request incrementally with one additional commit since the last revision:

  Remove unused imports

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3582/files
  - new: https://git.openjdk.java.net/jdk/pull/3582/files/b6bbadac..6ef93d7d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3582&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3582&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3582.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3582/head:pull/3582

PR: https://git.openjdk.java.net/jdk/pull/3582