I have no idea how it happened that \B, \H, \h, \L, and \l got

I have no idea how it happened that \B, \H, \h, \L, and \l got
mapped to ESCAPE_NUMBERED (which is for \N and only for \N), that
made no sense at all.  Properly remap them to ESCAPE_IGNORE.

While here, move \B and \w from the group taking number arguments
to the group taking string arguments; right now, that doesn't imply
any functional change, but if we ever go ahead and implement a
parser for roff(7) numerical expressions, it will suddenly start
to matter, and cause confusion.