Add support for help and version info.
Use new parse_mask() function for -m/--mask option.
(longopts): Add long options structure.
(opts): Move options string from getopts call to static var.
(usage): Print usage information.
(SCCSid): Version info.
(version): New function for displaying version info.
(parse_mask): New function supporting parsing of mnemonics,
hex, and basic expressions in masks.
(mnemonic2ul): New mnemonic parsing function.
(tag_mask_mnemonic): New type.
(mnemonic_table): New table of mnemonics for mnemonic2ul() to
search through.
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