Enum improvements for ad9361 tool

- Refactor string/value mapping and expose from library layer
- Add lookup calls to library for user convenience
- Add enum convenience macros for user commands
- Add extra help function to expand enums in online help
- Improve validation of numbers, expand enums for invalid values
- Refactor whitespace trimming into common layer
- Fix an output bug in format_int resulting in 1 for all nonzero values
- Fix output variable name for get_frequency_division_duplex_mode_enable
- Implement get_bist_timing_analysis
- Remove vestigial parse_SINT16()