xkbcli how-to-type: Enhance arguments parsing & doc

[wismill]

Currently the positional parameter of the CLI is either a Unicode code
point or a keysym. However their respective format is not documented.

It turns out that there are multiple issues due to the use of strtol:

• Code points can be parsed as octal, decimal and hexadecimal while
  keysyms can only be parsed as hexadecimal. Some programs outputs
  keysyms in their decimal form (e.g. wev) so it is worth to bring
  symmetry with code points.
• Octal format is unusual for both and is triggered by leading zeros,
  which is unintuitive in this context.
• U+NNNN format is the standard format for Unicode code points but is
  not supported.
• Plain characters are not supported, e.g.: a, é, ß, Æ, γ, 🦆, etc.
  Although this is probably the easiest format for most users.

Fixed the issues above:

• Allow the code point to be passed exactly in the following formats:
  • Literal character (requires UTF-8 character encoding of the terminal);
  • Decimal number;
  • Hexadecimal number: either 0xNNNN or U+NNNN.
• Allow the keysym to be passed exactly in the following formats:
  • Decimal number;
  • Hexadecimal number: either 0xNNNN;
  • Name.
• Improve both --help message and manual.

[wismill]

Hmm the use of strtol with base 0 parses strings starting with 0 as octal numbers. We should probably strip leading zeros to parse only decimal and hexadecimal numbers. It’s too easy to copy a Unicode code point from the standard format (e.g. U+0061) and get unexpected result.

[wismill]

A big part of the new code is about UTF-8 to UTF-32 decoding. It’s kept as an internal API for now. I added the related tests.

[wismill]

Note that we could go further and try to parse a keysym name without the --keysym flag, if previous attempts to parse as a code point / plain character failed. Maybe a new flag --codepoint would be useful then for completeness.