Customized version of munpack

When the attachment's suggested filename has spaces or other undesirable characters, munpack replaces them with 'X'. ThisXisXugly, and I'd prefer to let the user specify a replacement character. Here is a patch that adds an option [-r character] so that munpack -r- would produce This-is-better. The default is still 'X', so as not to disrupt any expectations. The manual page and usage string have been updated accordingly.