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.