Change streq to return bool

Change streq to return bool

I wanted to use streq with std::unique in another (upcoming) patch in
this seres, so I changed it to return bool.  To my surprise, this lead
to regressions.  The cause turned out to be that streq was used as an
htab callback -- by casting it to the correct function type.  This
sort of cast is invalid, so this patch adds a variant which is
directly suitable for use by htab.  (Note that I did not add an
overload, as I could not get that to work with template deduction in
the other patch.)

ChangeLog
2018-04-05  Tom Tromey  <tom@tromey.com>

	* completer.c (completion_tracker::completion_tracker): Remove
	cast.
	(completion_tracker::discard_completions): Likewise.
	* breakpoint.c (ambiguous_names_p): Remove cast.
	* ada-lang.c (_initialize_ada_language): Remove cast.
	* utils.h (streq): Update.
	(streq_hash): Add new declaration.
	* utils.c (streq): Return bool.
	(streq_hash): New function.