util.c: printutf8pad(): improve padded printing and printing invalid unicode characters

This affects sfeed_plain.

- Use unicode replacement character (codepoint 0xfffd) when a codepoint is
  invalid and proceed printing the rest of the characters.

- When a codepoint is invalid reset the internal state of mbtowc(3), from the
  OpenBSD man page:

  "  If a call to mbtowc() resulted in an undefined internal state, mbtowc()
     must be called with s set to NULL to reset the internal state before it
     can safely be used again."

- Optimize for the common ASCII case and use a macro to print the character
  instead of a wasteful fwrite() function call. With 250k lines (+- 350MB) this
  improves printing performance from 1.7s to 1.0s on my laptop. On an other
  system it improved by +- 25%.  Tested with clang and gcc and also tested the
  worst-case (non-ASCII) with no penalty.

To test:

	printf '0\tabc\xc3 def' | sfeed_plain

Before:

  1970-01-01 01:00  abc

After:

  1970-01-01 01:00  abc� def

0 files changed, 0 insertions, 0 deletions