Support silent shellcode injection into DLLs

Only run code on DLL_PROCESS_ATTACH, preventing infinite loop otherwise:
Added code would create thread -> calls DLL entry point -> calling added code...
Mitigate metasm relocation error by disabling ASLR
Deal with import error by actually using the GetProcAddress code.
Removing unused endjunk, sections_end, cert_entry
Merge pull request #1103 from scriptjunkie/dllinjectfix

Support silent shellcode injection into DLLs