getopt: merge from gnulib: alloca avoidance

getopt: merge from gnulib: alloca avoidance

In one place, glibc's getopt uses alloca to construct a linked list of
possibilities for an "ambiguous" long option.  In gnulib, malloc
should be used instead.  Providing for both cases complicates things a
fair bit.  Instead of merging straight across, therefore, I have
chosen to rewrite it using a boolean vector instead of a linked list.
There is then only one allocation that might need freeing; in glibc it
can honor __libc_use_alloca as usual, and in gnulib we define
__libc_use_alloca to always be false, so we don't need ifdefs in the
middle of the function.  This should also be slightly more efficient
in the normal case of long options being fully spelled out -- I think
most people aren't even aware they _can_ sometimes abbreviate long
options.

One interesting consequence is that the list of possibilities is now
printed in exactly the order they appear in the list of long options,
instead of the first possibility being shuffled to the end.  This
shouldn't be a big deal but it did break one test that relied on the
exact text of this error message.

(The reason the previous patch was "in aid of" merging from gnulib is
I didn't want to have to make this change in two places.)

(The patch looks bigger than it really is because there's a fair bit
of reindentation and code rearrangement.)

	* posix/getopt.c: When used standalone, define __libc_use_alloca
	as always false and alloca to abort if called.
	(process_long_option): Rewrite handling of ambiguous long options
	to use a single boolean vector, not a linked list; use
	__libc_use_alloca to decide whether to allocate this using alloca.

	* posix/tst-getopt_long1.c: Adjust text of expected error message.