shellcheck: coverage script issues

Shellcheck (​​https://github.com/koalaman/shellcheck) finds the following issues:

In coverage line 11:
    BN=`basename $fn`
       ^-- SC2006: Use $(..) instead of legacy `..`.
                 ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 12:
    DN=`dirname $fn`
       ^-- SC2006: Use $(..) instead of legacy `..`.
                ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 13:
    F=`echo $BN | sed -e 's/\.c$//;'`
      ^-- SC2006: Use $(..) instead of legacy `..`.
            ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 16:
    ONS=`echo ${DN}/src_*-${F}.o`
        ^-- SC2006: Use $(..) instead of legacy `..`.
              ^-- SC2086: Double quote to prevent globbing and word splitting.
                          ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 23:
        GCNO=`echo $on | sed -e 's/\.o$/\.gcno/;'`
             ^-- SC2006: Use $(..) instead of legacy `..`.
                   ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 24:
        if [ -e $GCNO ]
                ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 28:
          rm -f $GC
                ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 29:
          gcov -o $on $fn
                  ^-- SC2086: Double quote to prevent globbing and word splitting.
                      ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 30:
          if [ -e $GC ]
                  ^-- SC2086: Double quote to prevent globbing and word splitting.


In coverage line 34:
              mv $GC $dst/$GC
                 ^-- SC2086: Double quote to prevent globbing and word splitting.
                     ^-- SC2086: Double quote to prevent globbing and word splitting.
                          ^-- SC2086: Double quote to prevent globbing and word splitting.