gmtp: pass --datapath in the wrapper

gmtp gets somewhat confused trying to lookup some of its own assets when
installed system-wide. It first attempts to locate the path to its own binary by
searching for itself in $PATH, arrives at /run/current-system/sw/bin/gmtp, and
then performs lookups relative to that path without dereferencing the
symlink. Some of the lookups result in searching for, for example, icons in
/run/current-system/sw/bin/../share/gmtp/ which doesn't exist.