Don't try to write very long strings to portable files. The format

Don't try to write very long strings to portable files.  The format
does not support it.

(MAX_POR_WIDTH): New macro.
(pfm_open_writer): Limit output width to MAX_POR_WIDTH.
(write_format): Add arg to take width to resize format to.
(write_value): Limit width of value written to MAX_POR_WIDTH.
(write_variables): Limit width of variable and its output formats
to MAX_POR_WIDTH.