dlopen/dlsym support committed

dlopen/dlsym support committed

[Rich Felker]

Greetings!

I've committed experimental dlopen/dlsym support for musl. Presently
they only work if the main program is dynamic linked. Making them work
for static linked programs actually requires some really ugly hackery
which I'm putting off for the time being.

Some other limitations:

- dlclose is a no-op. There's not yet any way to unload libraries
  which are no longer needed. This should eventually be possible
  however, at least for RTLD_LOCAL mode.

- dlerror is a dummied out, so it's very hard to tell what went wrong
  if loading or resolving a symbol fails.

Nonetheless, I believe even the current very limited version meets the
POSIX requirements for how dependency and symbol resolution should
take place, and it should be possible to compile and use programs that
depend on dynamic module loading.

I'd be happy to see some reports of success or failure with major
software packages.

Rich