libc: make dlopen() return main program for empty file name

This is not a documented behavior but glibc's dlopen behaves this way.

Spotted when debugging assertion failure when java is started with a
debugger, we try to read elf header from "/" on dlopen("", ...):

Assertion failed: data.uio_resid == 0 (osv/fs/fs.cc: read: 27)

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

libc/dlfcn.cc

@@ -35,7 +35,7 @@ static void dlerror_fmt(const char *fmt, ...)
 
 void* dlopen(const char* filename, int flags)
 {
-    if (!filename) {
+    if (!filename || !strcmp(filename, "")) {
         // It is strange that we are returning a program while
         // dlsym will always try to open an object. We may have to
         // revisit this later, specially if this affect the ordering