Add automatic downloading of debug symbols and patch elf command

[astro-angelfish]

This PR is like #39. But I've finished automatic downloading of all platforms.
I've re-organized the structure of the libc database, allowing using debug symbols in different libc with different ID, and also allowing to save other shared object files like ld.so in libc packages for other exploitation like heap exploitation, which is requested in #9 .

The added code has also not been extensively tested. It might work on my machine, but bash is notoriously easy to mess up.
I will test it after the downloading of the libc database. It might not fit into the https://libc.rip website
Patching command currently does not support multiple libcs under the same directory. Maybe I need to add interaction to the script later.
I've added the support of the multiple libraries under the same directory.

I'm not a native English speaker so there might be grammatical errors or typos in the script or this PR message.

After this PR, all of the libc databases need to be re-downloaded to fit into the new database structure
Please wait for the repairing of compatibility of gdb. I have little time to do it because I'm busy with my study.

[astro-angelfish]

Sorry for not updating because I forgot what I did and there is still some issues.