grep: fix -Fh

[mknos]

• When combining grep -F and -h flags, the filenames were not listed for matches (correct), but they still were not listed when unsetting -h (incorrect)
• test1: "perl grep -F '/' awk ar" --> output should show "$filename:" prefix
• Init of global variable $Mult was missed due to early return in parse_args()
• I also tested that -Fl works as expected
• While here, remove the need for variable $optstring by passing options directly to getopts()

[briandfoy]

changes: show filenames even without -h set