center-v1.1.0

The following changes have been made since v1.0.0:

Makefile changes:
  - Use the MANDIR variable.
  - Compile with -fomit-frame-pointer, and some other CFLAGS cleanups.

center changes:
  - Try to read display width from $COLUMNS before falling back to
    ioctl().
  - Fixed the incorrect diagnostic printed when the display width value
    overflows an integer.
  - If we can’t figure out the screen width, default to 80.
  - Properly handle backspace characters (‘\b’) so that we can do things
    like:
      $ mandoc center.1 | center -lw $(tput cols) | less
    and get expected output.
  - Various code changes
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