pragma: handle default name/fallback handler for NULL list; cleanups

pragma: handle default name/fallback handler for NULL list; cleanups

The previous code would fail to process any directive if the directive
list was NULL. However, we also need to process the default name
passed to search_pragma_list() (e.g. "elf32"), as well as the global
name (e.g. "output") and call the default handler in that case.

In the process, improve the handling such that if one handler returns
DIRR_UNKNOWN, try calling subsequent handlers in the list.

Finally, factor out as much as possible to generic handlers.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>