lib/string/strspn/, lib/, src/: stprspn(), strrspn_(): Split API into function and...

lib/string/strspn/, lib/, src/: stprspn(), strrspn_(): Split API into function and macro

This provides a safer and more consistent API.

We had the strrspn(3) function as it was for compatibility with Oracle
Solaris, but let's not repeat their mistake.  Nevertheless, name our
function strrspn_() with a trailing underscore, to differentiate it from
the one in Solaris, since it's slightly different.

Signed-off-by: Alejandro Colomar <alx@kernel.org>