READ-BDF: Handle newly encountered cases in BDF files. (#2108)

* Better handling of a glyph with ENCODING of -1. 
I treat it as the _slug_ glyph, instead of the _default_ of a solid block.

* Handle scrambled bitmaps issue #2109.
Glyphs with zero width bitmap *and* zero advance (_escapement_) caused miscalculated glyph offsets into the CHARSETINFO bitmap.

* Allow and ignore COMMENT lines preceding the STARTFONT line.
Add error checking for extracting font FAMILY, SIZE, FACE, etc. from the BDF-FONT object.
Add recommendation to documentation to write the DISPLAYFONT files to a directory separate from the system's IL:DISPLAYFONTDIRECTORIES locations.

* Account for glyphs with a negative initial offset.

* Add VERBOSE optional parameter to READ-BDF to report font internal FAMILY, FACE, etc.
Change &OPTIONAL parameters of WRITE-BDF-TO-DISPLAYFONT-FILES to &KEY to simplify calling. (No need to remember the order.)
Add CHAR-SETS and WRITE-UNMAPPED parameters to WRITE-BDF-TO-DISPLAYFONT-FILES to allow some level of control of which DISPLAYFONT files are written.
Updated documentation, and added warning note about font's FAMILY containing any digits.

Author: MattHeffron