libctf: rename ctf_dict.ctf_{symtab,strtab}

These two fields are constantly confusing because CTF dicts contain both
a symtypetab and strtab, but these fields are not that: they are the
symtab and strtab from the ELF file.  We have enough string tables now
(internal, external, synthetic external, dynamic) that we need to at
least name them better than this to avoid getting totally confused.
Rename them to ctf_ext_symtab and ctf_ext_strtab.

libctf/

	* ctf-dump.c (ctf_dump_objts): Rename ctf_symtab -> ctf_ext_symtab.
	* ctf-impl.h (struct ctf_dict.ctf_symtab): Rename to...
	(struct ctf_dict.ctf_ext_strtab): ... this.
	(struct ctf_dict.ctf_strtab): Rename to...
	(struct ctf_dict.ctf_ext_strtab): ... this.
	* ctf-lookup.c (ctf_lookup_symbol_name): Adapt.
	(ctf_lookup_symbol_idx): Adapt.
	(ctf_lookup_by_sym_or_name): Adapt.
	* ctf-open.c (ctf_bufopen_internal): Adapt.
	(ctf_dict_close): Adapt.
	(ctf_getsymsect): Adapt.
	(ctf_getstrsect): Adapt.
	(ctf_symsect_endianness): Adapt.