x11/bosd: Add EXAMPLES option

Install the sample glyph scripts only when EXAMPLES is on. The
glyph PNGs stay in the icon directory.

Suggested by: fuz (mentor)
Approved by: fuz (mentor), jrm (mentor)
Reviewed by: fuz (mentor), jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D59989