SSV cleanups and refactorings

The old and new plugins' SSV implementations need to be refactored after the changes for 6581221 are put back. The code now in ssv_common needs to be updated with the modifications made in 6581221, and the code for ssv.dll needs to be changed to use the code in ssv_common.
In particular, the code to delete the old plugins out of the Firefox plugins directory needs to be pulled in, and the GetLoadedModuleDirectory.[ch] just added to the workspace need to be deleted in favor of the version in common.cpp.
The code in the installer needs to be updated to call into the right ssv.dll or jp2ssv.dll to put in place the CLSID registry keys (for the case where we are doing an installation while the browser is still open). This code exists in 6u4, and was just brought forward to 6u5. It is also necessary to do HLKM key redirection as we have found that Vista consults HKLM and not HKCU when UAC is disabled (see 6612714). We will probably need to change the common entry point exposed from ssv.dll and jp2ssv.dll to indicate that the call is being made from a high integrity process (on Vista). We will also likely need to provide the name of the SSV broker executable, or probably more preferably merge the two SSV brokers into one.