MFC direct execution mode for rtld.

MFC r318313:
Make ld-elf.so.1 directly executable.

MFC r318352 (by jonathan):
Rename rtld's parse_libdir to parse_integer.

MFC r318380:
Pretend that there is some security when executing in direct mode.

MFC r318431 (by jonathan):
Allow rtld direct-exec to take a file descriptor.

MFC r318445:
Fix style, add static keyword before static function definition.

MFC r318739:
For ld.so direct execution mode, implement -p option.