Perform the following ELF security checks:
- PIE: Check for position independent executable (PIE), allowing for address space randomization
- NX: Check that no sections are writable and executable (including the stack)
- RELRO: Check for read-only relocations, binding at startup
- Canary: Check for use of stack canary
Also add a check to symbol-check.py that checks that only the subset of
allowed libraries is imported (to avoid incompatibilities).
Add a script to check that the (Linux) executables produced by gitian
only contain allowed gcc, glibc and libstdc++ version symbols. This
makes sure they are still compatible with the minimum supported Linux
distribution versions.
