gas/ELF: allow "inheriting" section attributes and type

While --sectname-subst is nice, it isn't enough to e.g. mimic
-f{function,data}-sections in assembly code, when such use is to be
optional (e.g. dependent upon some configuration setting).

Assign meaning to '+' and '-' as section attribute letters, allowing
to inherit the prior section's attributes (and possibly type) along
with adding or removing some. Note that documenting the interaction
with '?' as undefined is a precautionary measure.

While touching the function invocation, stop using |= on the result of
obj_elf_parse_section_letters(): "attr" is firmly zero ahead of the
call.