write: correctly handle wide characters

write: correctly handle wide characters

Do this by replacing fputc_careful() (notice that the description said
it's locale-aware ‒ it very much is /not/), with a fputs_careful() which
does the same thing, but if it were to output a byte in the \123 format,
first it checks whether this byte starts a valid multibyte character.

If it does, and that character is printable, write it verbatim.
This means that
  echo 'foo åäö ąęćźżń bar' | write nabijaczleweli pts/4
instead of
  foo \303\245\303\244\303\266
  \304\205\304\231\304\207\305\272\305\274\305\204 bar
yields
  foo åäö ąęćźżń bar
or, more realistically, from a message I got earlier today,
  Filip powiedzia\305\202 \305\274e zap\305\202aci jutro
becomes
  Filip powiedział że zapłaci jutro

Invalid/non-printable sequences get processed as before.

Line reading in write must become getline() to avoid dealing with
partial characters: for example on input consisting solely of
ąęćźżń, where every {1} is an instance, the output would be
  {42}ąęć\305\272żń{84}ąęćź\305\274ń{84}ąęćźż\305\204{39}
with just fixed-512 fgets()

Bug-Debian: https://bugs.debian.org/826596