Dwarf: adjust fetch_indirect{,_line}_string()'s return type

Dwarf: adjust fetch_indirect{,_line}_string()'s return type

As already is the case for fetch_indexed_string() and
fetch_alt_indirect_string(), have them return a pointer to plain char,
properly compatible with e.g. the %s format specifier that the return
values are used with in several cases. This way no casts at call sites are
necessary, and several (bogus) ones in the functions can be exchanged for
just one each. Some other constification is needed elsewhere, now that
const-ness isn't being cast away anymore.