dracut.sh: fix errors pointed out by shellcheck.

dracut.sh: fix errors pointed out by shellcheck.

- use [ ] instead of calling test manually, as most of the script is
doing.

- use quotes in hostonly_cmdline blocks, specially when dealing with the
conf files, whose names are set by users/system administrators

- uefi_splash_image was being assigned set to `${dracutsysroot}...`, which
is a variable that doesn't exist.

- we don't want the conditional to run the output of fsfreeze as
commands. Instead, we just need to know if any of the fsfreeze commands
failed.