Command-line argument parsing for built-ins (POSIX portability)

[magicant]

Features for non-portable extensions are tracked in #321.

• Basics Built-in argument syntax parser basics #125
  • Short options
  • Long options
  • Argument-taking options
  • Separator (--)
• Syntax error message 14b1475
  • Wrong option
  • Ambiguous long option
  • Missing option argument
  • Unexpected long option argument
• Mode: enabling/disabling long options Argument parser mode for rejecting long options #127
• Show the built-in name in error messages
• Utility for reporting conflicting options Handle conflicting options in unset built-in #303