bstr: change bstr0_s to bstr0_lit, restrict it only for string literals

Allows to avoid non-portable strlen usage. Also avoid "initializer
element is not constant" warnings on older GCC that doesn't like
explicit type specification in aggregate initialization.

Co-authored-by: NRK <nrk@disroot.org>
Co-authored-by: nanahi <130121847+na-na-hi@users.noreply.github.com>