libctf: do not deduplicate strings in the header

libctf: do not deduplicate strings in the header

It is unreasonable to expect users to ctf_import the parent before being
able to understand the header -- doubly so because the only string in the
header which is likely to be deduplicable is the parent name, which is the
same in every child, yet without the parent name being *available* in the
child's strtab you cannot call ctf_parent_name to figure out which parent
to import!

libctf/
	* ctf-serialize.c (ctf_preserialize): Prevent deduplication of header string
        fields.
	* ctf-open.c (ctf_set_base): Note this.
	* ctf-string.c (ctf_str_free_atom): Likewise.