Before this change, the systemd meter was broken on distros like NixOS,
which have systemctl in PATH, but not at /bin/systemctl. After the
change, it works on all my NixOS machines.
`RichString_appendnAscii()` avoids a `strlen(3)` call over
` RichString_appendAscii()`.
Use the former where the length is available from a previous checked
`snprintf(3)` call.
Keep `RichString_appendAscii()` when passing a string literal and
rely on compilers to optimize the `strlen(3)` call away.
RichString_writeFrom takes a top spot during performance analysis due to the
calls to mbstowcs() and iswprint().
Most of the time we know in advance that we are only going to print regular
ASCII characters.
