For termux/termux-api#158, replacing #12

Significant changes:

1. Avoid all the unnecessary "flags" with a single record variable.
2. Use an array for params so that proper quoting can be done
3. Use getopts builtin error reporting (for both illegal option and missing required argument) but show usage additionally
4. Avoid redundant code by using a function sole() for options that should be solely used; tolerate -d to be used with -f or -l as in the original script
5. use dirname + realpath instead of pwd for possible absolute path (e.g. /sdcard/somedir/somefile)