ICU-20545 Detect file separator char from dir

ICU-20545 Detect file separator char from dir

If udata_create won't find U_FILE_SEP_CHAR at the end of a dir variable,
then it appends it. The problem starts if the dir variable uses
U_FILE_ALT_SEP_CHAR which is not equal to U_FILE_SEP_CHAR. Then the
resulting path could look like this
../data\mappings/cns-11643-1992.ucm
instead of this
../data/mappings/cns-11643-1992.ucm

This patch uses U_FILE_SEP_CHAR unless it detects that the dir variable
doesn't use it, and uses U_FILE_ALT_SEP_CHAR instead.