Quotes and {} audit for variables and strings

Use single quotes, double quotes, and braces when they're supposed to be
used.

Switched many double quotes to single quotes.
Removed many unneeded braces.
Removed quotes when assigning a variable to a variable, as field
splitting is not done during variable assignment
(http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06_05)
Use slightly more compact format for arithmetic