fileio: clean up write_string_file() naming

fileio: clean up write_string_file() naming

let's rename the "_ts" flavour of these calls "_full" instead, exposing
the full functionality. And then keep two more minimal versions around:
one "_at" (which has the ts parameter suppressed, but keeps the dir_fd
one). And one without suffix (which supresses both).

Do the same for the label versions of these calls.