SORT_BY_INIT_PRIORITY

SORT_BY_INIT_PRIORITY

I was looking at the implementation of this script keyword today and
couldn't remember why we do what we do in get_init_priority, because
the comments explain how the init_priority is encoded but don't say
why it is necessary to extract the priority and sort on that.  So
after figuring out why (again), I wrote some more comments.

Then I simplified get_init_priority a little, adding some sanity
checking on the strtoul result.  This actually makes get_init_priority
support sorting by numerical suffix more generally, but I figure this
feature would be better as a new keyword (without the .ctors/.dtors
special case), so haven't documented the extension.

	* ld.texi (SORT_BY_ALIGNMENT): Reword slightly.
	(SORT_BY_INIT_PRIORITY): Elucidate.
	* ldlang.c: Include limits.h.
	(get_init_priority): Comment.  Change param to a section,
	return an int.  Sanity check priority digits.  Support sorting
	more sections with trailing digits.  Return -1 on error.
	(compare_section): Adjust.