On DragonFly use procfs for self_exe_name()

Using /proc/curproc/file does not resolve mount-points as sysctl(3) with
KERN_PROC_PATHNAME does, which gave back paths containing colons like
/pfs/@@-1:00004/usr/bin. This led to problems when joining paths based on it.

Author: mneumann

src/libstd/os.rs

@@ -665,7 +665,7 @@ pub fn dll_filename(base: &str) -> String {
 /// ```
 pub fn self_exe_name() -> Option<Path> {
 
-    #[cfg(any(target_os = "freebsd", target_os = "dragonfly"))]
+    #[cfg(target_os = "freebsd")]
     fn load_self() -> Option<Vec<u8>> {
         unsafe {
             use libc::funcs::bsd44::*;
@@ -691,6 +691,16 @@ pub fn self_exe_name() -> Option<Path> {
         }
     }
 
+    #[cfg(target_os = "dragonfly")]
+    fn load_self() -> Option<Vec<u8>> {
+        use std::io;
+
+        match io::fs::readlink(&Path::new("/proc/curproc/file")) {
+            Ok(path) => Some(path.into_vec()),
+            Err(..) => None
+        }
+    }
+
     #[cfg(any(target_os = "linux", target_os = "android"))]
     fn load_self() -> Option<Vec<u8>> {
         use std::io;