security: Round 6.3 shell command substitution fix (A18)

security: Round 6.3 shell command substitution fix (A18)

A18: Fix shell command substitution  syntax
- expand_vars was adding '(' at the start but missing ')' at the end
- Added closing parenthesis to properly wrap subshell command
- Changed cmd[1 + cmdlen] = '\0' to cmd[1 + cmdlen] = ')' and cmd[2 + cmdlen] = '\0'

Tests: 119/119 PASS (smoke test, SMP=4)