Set NODEFLIB in user app to prevent use of target system libs

[JonathonReinhart]

This will hopefully avoid an entire class of problems where libraries from the target environment are inadvertently loaded.

This works not only for NEEDED libraries, but also calls to dlopen().

Closes #137

[JonathonReinhart]

This was too good to be true. It turns out NODEFLIB only applies to the object whose dependencies are being resolved; not dependencies of those dependencies.

Evidence:

• ld(1) man page says:

  nodefaultlib
  Specify that the dynamic loader search for dependencies of this object should ignore any default library search paths.

• GLIBC source code is confusing as hell. I don't know what l and loader are
• freebsd rtld very clearly stores z_nodefaultlib per-object and always evaluates it in such a context.
• I wrote a test (TODO: post a Gist) where app calls dlopen("./libfoo.so") which depends on libcrypt.so, and I set -z nodefaultlib on app. The flag woks, because I needed to set RPATH=$ORIGIN and copy libc.so and libdl.so to even try to run app (as expected). But libcrypt from the system paths was loaded ☹️

  [app:main] Started
  [app:main] my dir: /home/jreinhart/quicktests/nodeflib
  [app:main] libfoo path: /home/jreinhart/quicktests/nodeflib/libfoo.so
       10531: find library=libcrypt.so.1 [0]; searching
       10531:  search cache=/etc/ld.so.cache 
       10531:   trying file=/lib/x86_64-linux-gnu/libcrypt.so.1