Fixed various Undefined Behavior occurrences.

Conditionally uninitialized:
 - builtin_commandline.cpp:577
 - expand.cpp:869
 - parse_util.cpp:1036

Initialization of POD structs:
 - event.cpp:61
 - autoload.cpp:22

References used with va_start:
 - common.cpp:608:18

Found with clang-3.4's awesome -Wconditional-uninitialized,
-Wmissing-field-initializers and -Wvarargs.

autoload.cpp

@@ -19,7 +19,7 @@ static const int kAutoloadStalenessInterval = 15;
 file_access_attempt_t access_file(const wcstring &path, int mode)
 {
     //printf("Touch %ls\n", path.c_str());
-    file_access_attempt_t result = {0};
+    file_access_attempt_t result = {};
     struct stat statbuf;
     if (wstat(path, &statbuf))
     {