Fix some shell issues by adding quotes in fwget.sh

[bcr]

Summary

This patch fixes the following issues in the shell script:

• Quote default assignments, see SC2223
• Use $() instead of backticks, see SC2006
• Double quote $@, see SC2068
• Double quote variables in if-statements and other places, see SC2086

While here, fix a whitespace at end of line instance in the license text.

Test Plan

1. Apply the patch to /usr.sbin/fwget
2. Run fwget to see if it still works