Add options for finding temp directory

On Ubuntu, $TMPDIR is not set. So, add an option to try
/var/tmp or /tmp directories.

src/corehost/common/pal.h

@@ -193,8 +193,8 @@ namespace pal
     inline bool pal_clrstring(const pal::string_t& str, std::vector<char>* out) { return pal_utf8string(str, out); }
     inline bool clr_palstring(const char* cstr, pal::string_t* out) { out->assign(cstr); return true; }
 
-    inline bool mkdir(const pal::char_t* dir, int mode) { return mkdir(dir, mode) == 0; }
-    inline bool rmdir(const pal::char_t* path) { return rmdir(path) == 0; }
+    inline bool mkdir(const pal::char_t* dir, int mode) { return ::mkdir(dir, mode) == 0; }
+    inline bool rmdir(const pal::char_t* path) { return ::rmdir(path) == 0; }
     inline int rename(const pal::char_t* old_name, const pal::char_t* new_name) { return ::rename(old_name, new_name); }
     inline int remove(const pal::char_t* path) { return ::remove(path); }
     inline int get_pid() { return getpid(); }

src/corehost/common/pal.unix.cpp

@@ -182,8 +182,30 @@ bool pal::get_default_servicing_directory(string_t* recv)
 
 bool pal::get_temp_directory(pal::string_t& tmp_dir)
 {
-    pal::getenv(_X("TMPDIR"), &tmp_dir);
-    return pal::realpath(&tmp_dir);
+    // First, check for the POSIX standard environment variable
+    if (pal::getenv(_X("TMPDIR"), &tmp_dir))
+    {
+        return pal::realpath(&tmp_dir);
+    }
+
+    // On non-compliant systems (ex: Ubuntu) try /var/tmp or /tmp directories.
+    // /var/tmp is prefered since its contents are expected to survive across
+    // machine reboot.
+    pal::string_t _var_tmp = _X("/var/tmp/");
+    if (pal::realpath(&_var_tmp))
+    {
+        tmp_dir.assign(_var_tmp);
+        return true;
+    }
+
+    pal::string_t _tmp = _X("/tmp/");
+    if (pal::realpath(&_tmp))
+    {
+        tmp_dir.assign(_tmp);
+        return true;
+    }
+
+    return false;
 }
 
 bool pal::get_global_dotnet_dirs(std::vector<pal::string_t>* recv)

src/corehost/corehost.cpp

@@ -131,15 +131,15 @@ int exe_start(const int argc, const pal::char_t* argv[])
         return StatusCode::AppHostExeNotBoundFailure;
     }
 
-    app_root.assign(app_path);
-
     append_path(&app_path, embedded_app_name.c_str());
     if (!pal::realpath(&app_path))
     {
         trace::error(_X("The application to execute does not exist: '%s'."), app_path.c_str());
         return StatusCode::LibHostAppRootFindFailure;
     }
 
+    app_root.assign(get_directory(app_path));
+
 #else
     pal::string_t own_name = strip_executable_ext(get_filename(host_path));