Exposes libc::dlopen flags for unix platform.

[whereskenneth]

I have a requirement for passing the RTLD_NOW flag to libc::dlopen.

This situation arises when undefined symbols in a shared library are present. The undefined symbols are ultimately defined, but in another library that is linked into the library being loaded.

For example:

$ ldd a.so
# a.so links to b.so and c.so
b.so
c.so

$ nm -D b.so
# Foo is undefined in b.so
U  Foo

$ nm -D c.so
# Foo later defined in c.so
0000000008 T Foo

Using default flags of RTLD_LOCAL | RTLD_LAZY, for some reason, causes Foo to not be found during runtime.

Testing revealed LD_BIND_NOW=1 works to override this behavior, but that is unfeasible for my use case. I need to be able to specify RTLD_NOW as a flag when callinglibc::dlopen.

Therefore, I augmented the public interface to support opening a library with flags: Option<i32> . Passing None preserves the current behavior. This is a non-breaking change to the public API as far as I can tell.