bootparamd: Fix several warnings and increase warn level to 6.

• Increase WARNS to 6.
• Except -Wcast-align and -Wincompatible-pointer-types-discards-qualifiers checks.
• Use ANSI C prototype.
• Statically variables and functions.
• Add extern declaration for global variables.
• Rename local variables to resolve shadow warnings.

PR: 71667

(cherry picked from commit e03764d931d820185a019334259b18df2e3f6b6c)