Use symbolic link to avoid two appscope libraries in one process

[jrcheli]

On upgrade of appscope, when using rules, it's possible to have /etc/ld.so.preload refer to one version of the appscope library, and LD_PRELOAD refer to another. When this happens, the loader can load both versions of the appscope library into the same process.

This only occurs when the checksums of the libraries are different, but this will be the case when moving from v1.4.0 to v1.4.1, as an example.

When one process has multiple appscope libraries loaded into it, we've observed crashes in the memory subsystem. We believe this is due to each library using it's own memory subsystem; the crash occurring when memory is allocated by one, and an attempt is made to free it by the other.