cp: add \# escape sequence

cp: add \# escape sequence

\# is like \", but consumes the ending newline so that one can comment
complex input like this:
  \D't 0'\#                         set minimal drawing thickness
  \Z"\D'l 0 -\\n[nl]u-1v'"\#        left margin intersecting top margin
  \Z"\D'l 0 \\n[.t]u'\#             left margin
  \&\Z`\D'l 0 \\n[.p]u-\\n[nl]u'`\# left margin intersecting bottom margin
  \&\Z`\D'l -\\n[.o]u 0'`\#         bottom margin intersecting left margin
  \&\D'l \\n[.l]u+\\n[.o]u 0'"\#    bottom margin & intersection w/ right margin
  \Z"\D'l -\\n[.o]u 0'"\#           top margin intersecting left margin
  \Z"\D'l \\n[.l]u 0'\#             top margin
  \&\Z`\D'l \\n[.o] 0'`\#           top margin intersecting right margin
  \&\Z`\D'l 0 -\\n[nl]u-1v'`\#      right margin intersecting top margin
  \&\D'l 0 \\n[.p]u-\\n[nl]u'"\#    right margin & intersection w/ bottom margin
  \D't -1'\"                        restore regular drawing thickness

(Escape borrowed from groff.)