* html.cc [!_POSIX_VERSION]: Include limits.h and dirent.h or

* html.cc [!_POSIX_VERSION]: Include limits.h and dirent.h or
sys/dir.h.  Define NAME_MAX using MAXNAMLEN.  Include nonposix.h.
(file_name_max): New function.
(html_printer::make_new_image_name): If the filesystem doesn't
support file names longer than 14 characters, use a shorter
image_name string.
(html_printer::convert_to_image): Enlarge the size of buffer[] to
accomodate 2 temp file names plus some slack.  Don't put n at the
end of commands passed to system().  Redirect stderr to the null
device programmatically, not via the shell.  Use NULL_DEV, not
literal "/dev/null".  Print diagnostics if any calls to system()
failed.

irrespective of the boolean `margin_on'.  Fixes a divide by zero bug
and a column bug as reported by Steve Blinkhorn <steve@prd.co.uk>.