libctf, ld, binutils: add textual error/warning reporting for libctf

libctf, ld, binutils: add textual error/warning reporting for libctf

This commit adds a long-missing piece of infrastructure to libctf: the
ability to report errors and warnings using all the power of printf,
rather than being restricted to one errno value.  Internally, libctf
calls ctf_err_warn() to add errors and warnings to a list: a new
iterator ctf_errwarning_next() then consumes this list one by one and
hands it to the caller, which can free it.  New errors and warnings are
added until the list is consumed by the caller or the ctf_file_t is
closed, so you can dump them at intervals.  The caller can of course
choose to print only those warnings it wants.  (I am not sure whether we
want objdump, readelf or ld to print warnings or not: right now I'm
printing them, but maybe we only want to print errors?  This entirely
depends on whether warnings are voluminous things describing e.g. the
inability to emit single types because of name clashes or something.
There are no users of this infrastructure yet, so it's hard to say.)

There is no internationalization here yet, but this at least adds a
place where internationalization can be added, to one of
ctf_errwarning_next or ctf_err_warn.

We also provide a new ctf_assert() function which uses this
infrastructure to provide non-fatal assertion failures while emitting an
assert-like string to the caller: to save space and avoid needlessly
duplicating unchanging strings, the assertion test is inlined but the
print-things-out failure case is not.  All assertions in libctf will be
converted to use this machinery in future commits and propagate
assertion-failure errors up, so that the linker in particular cannot be
killed by libctf assertion failures when it could perfectly well just
print warnings and drop the CTF section.

include/
	* ctf-api.h (ECTF_INTERNAL): Adjust error text.
	(ctf_errwarning_next): New.
libctf/
	* ctf-impl.h (ctf_assert): New.
	(ctf_err_warning_t): Likewise.
	(ctf_file_t) <ctf_errs_warnings>: Likewise.
	(ctf_err_warn): New prototype.
	(ctf_assert_fail_internal): Likewise.
	* ctf-inlines.h (ctf_assert_internal): Likewise.
	* ctf-open.c (ctf_file_close): Free ctf_errs_warnings.
	* ctf-create.c (ctf_serialize): Copy it on serialization.
	* ctf-subr.c (ctf_err_warn): New, add an error/warning.
	(ctf_errwarning_next): New iterator, free and pass back
	errors/warnings in succession.
	* libctf.ver (ctf_errwarning_next): Add.
ld/
	* ldlang.c (lang_ctf_errs_warnings): New, print CTF errors
	and warnings.  Assert when libctf asserts.
	(lang_merge_ctf): Call it.
	(land_write_ctf): Likewise.
binutils/
	* objdump.c (ctf_archive_member): Print CTF errors and warnings.
	* readelf.c (dump_ctf_archive_member): Likewise.