Change all the POSTINSTALL 'mkfontdir' to 'mkfontdir -x .lev' although
the mkfontdir version on my system didn't think $(HACKDIR)/*.lev were
font files when I built without dlb.
Also change the PREINSTALL 'cp -n win/X11/nethack.rc ~/.nethackrc' to
keep going if it fails. The linux hints use 'cp -n' for sysconf but
since it is doing so for the playground directory and 'make install'
starts out by clearing away everything in that directory, it shouldn't
fail. But some extra bullet proofing there may be warranted. Only
the initial cp is protected against clobbering an existing file; the
ownership+permission fixups that are applied to the copy of original
file still get applied to an existing one.
travis recently changed linux default dist from trusty to xenial, and bionic is next
include an additional travis linux build under bionic to eliminate surprises there
add a parameter to mkfontdir under linux to prevent it form going after .lev files
