I wanted to use streq with std::unique in another (upcoming) patch in
this seres, so I changed it to return bool. To my surprise, this lead
to regressions. The cause turned out to be that streq was used as an
htab callback -- by casting it to the correct function type. This
sort of cast is invalid, so this patch adds a variant which is
directly suitable for use by htab. (Note that I did not add an
overload, as I could not get that to work with template deduction in
the other patch.)