Lazily copy libraries before modifying and rework add_library()

[JonathonReinhart]

This PR was motivated by noticing this line:

https://github.com/JonathonReinhart/staticx/blob/159fba33/staticx/api.py#L177

I think this could cause some symlinks to get missed if --strip was passed.

I decided to just re-write this code to be safer, and also only copy libraries lazily before we modify them.

The order of operations looks to be correct:

INFO:root:Processing library libnss_files.so.2 (/lib/x86_64-linux-gnu/libnss_files.so.2)
INFO:root:Adding Symlink libnss_files.so.2 => libnss_files-2.31.so
INFO:root:Copying /lib/x86_64-linux-gnu/libnss_files-2.31.so to /tmp/staticx-archive-4hx7iz7r/libnss_files-2.31.so
INFO:root:Stripping library /tmp/staticx-archive-4hx7iz7r/libnss_files-2.31.so
DEBUG:root:Running ['strip', '/tmp/staticx-archive-4hx7iz7r/libnss_files-2.31.so']
INFO:root:Adding /tmp/staticx-archive-4hx7iz7r/libnss_files-2.31.so as libnss_files-2.31.so