Rework domain choices in ctfread.c

Rework domain choices in ctfread.c

Another patch I am working on induced some failures in CTF tests.
Looking into this, I found that ctfread.c seems to largely work by
accident.  In particular, it often chooses the wrong domain for a
symbol.

In CTF, I believe there are 4 kinds of symbols: types, variables,
functions, and "data objects" (which IIUC may be either a variable or
a function).

ctfread.c was examining the type-kind of a variable and sometimes
treating one as a type.  add_stt_entries and
ctf_psymtab_add_stt_entries only ever used VAR_DOMAIN (but are called
for functions, which should be in FUNCTION_DOMAIN).  And
ctf_psymtab_type_cb sometimes used VAR_DOMAIN, but is only called for
types, and so should only ever use TYPE_DOMAIN or STRUCT_DOMAIN.

This patch cleans all this up, based on my understanding of the
situation.  This passes the existing tests, and also works with my
aforementioned yet-to-be-submitted patch as well.

Finally, I renamed new_symbol because it is only used for type
symbols.

Acked-By: Simon Marchi <simon.marchi@efficios.com>