* src/preproc/html/pre-html.cpp (write_upto_newline): Don't use

* src/preproc/html/pre-html.cpp (write_upto_newline): Don't use
`(*t)->next' without testing validity of `*t'.
(usage): Make it more readable.

Make html device run under both MS-DOS and Win32.

* src/preproc/html/pre-html.cpp (MAY_FORK_CHILD_PROCESS,
MAY_SPAWN_ASYNCHRONOUS_CHILD): New macros to control whether
spawn or fork+exec has to be used, and whether parent must sleep
until the child process terminates.  Used in
`char_buffer::run_output_filter'.
(copyofstdoutfd): Removed.
(char_buffer): Replace `write_file_html' and `write_file_troff'
member functions with `emit_troff_output' and `run_output_filter'.
The latter
(DEVICE_FORMAT, HTML_OUTPUT_FILTER, IMAGE_OUTPUT_FILTER): New
macros.
(replaceFd): Replaced with...
(set_redirection): New auxiliary function.
(waitForChild): Replaced with...
(save_and_redirect): New auxiliary function for.

(char_buffer::do_html, char_buffer::do_image): Simplified, using new
functions.