desktopToDarwinBundle: Implement %f and %u Exec field codes

`%f` and `%u` are used to signal the program only accepts a single file
or URI argument. I do not believe there's a way to signal this
information to macOS but it is possible the program really won't work if
multiple files are passed and it's possible the relative position of
`%i`, `%c` or `%k` matters. So we replace `%f` or `%u` with `$1`. That
way we only pass one file in the (possibly significant) position of the
field code.

pkgs/build-support/setup-hooks/desktop-to-darwin-bundle.sh

@@ -178,7 +178,8 @@ processExecFieldCodes() {
   local -r execNoKC="${execNoK/\%c/$(getDesktopParam "${file}" "Name")}"
   local -r icon=$(getDesktopParam "${file}" "Icon")
   local -r execNoKCI="${execNoKC/\%i/${icon:+--icon }${icon}}"
-  local -r exec="${execNoKCI// %[fFuU]}"
+  local -r execNoKCIfu="${execNoKCI/\%[fu]/\$1}"
+  local -r exec="${execNoKCIfu// %[FU]}"
   if [[ "$exec" != "$execRaw" ]]; then
     echo 1>&2 "desktopToDarwinBundle: Application bundles do not understand desktop entry field codes. Changed '$execRaw' to '$exec'."
   fi