This commit is another step to identify and document the maximum number
of logging messages. Bulk changes:

- Rename `conflicting-key-type` to `conflicting-key-type-merging-groups`.
  Giving more context in the name allow us to introduce
  `conflicting-key-type-definitions` later.
- Add conflicting-key-type-definitions
- Add conflicting-key-type-map-entry
- Add undeclared-modifiers-in-key-type
  Also improve the log messages.
- Add conflicting-key-type-preserve-entries
- Use XKB_ERROR_UNSUPPORTED_MODIFIER_MASK
- Add illegal-key-type-preserve-result
- Add conflicting-key-type-level-names
- Add duplicate-entry
- Add unsupported-symbols-field
- Add missing-symbols-group-name-index
- Use XKB_ERROR_WRONG_FIELD_TYPE
- Add conflicting-key-name
- Use XKB_WARNING_UNDEFINED_KEYCODE
- Add illegal-keycode-alias
- Add unsupported-geometry-section
- Add missing-default-section
- Add XKB_LOG_MESSAGE_NO_ID
- Rename log_vrb_with_code to log_vrb
- Use ERROR_WRONG_FIELD_TYPE & ERROR_INVALID_SYNTAX
- Add unknown-identifier
- Add invalid-expression-type
- Add invalid-operation + fixes
- Add unknown-operator
- Rename ERROR_UNKNOWN_IDENTIFIER to ERROR_INVALID_IDENTIFIER
- Add undeclared-virtual-modifier
- Add expected-array-entry
- Add invalid-include-statement
- Add included-file-not-found
- Add allocation-error
- Add invalid-included-file
- Process symbols.c
- Add invalid-value
- Add invalid-real-modifier
- Add unknown-field
- Add wrong-scope
- Add invalid-modmap-entry
- Add wrong-statement-type
- Add conflicting-key-symbols-entry
- Add invalid-set-default-statement

Previously we had two types of macros for logging: with and without
message code. They were intended to be merged afterwards.

The idea is to use a special code – `XKB_LOG_MESSAGE_NO_ID = 0` – that
should *not* be displayed. But we would like to avoid checking this
special code at run time. This is achieved using macro tricks; they
are detailed in the code (see: `PREPEND_MESSAGE_ID`).

Now it is also easier to spot the remaining undocumented log entries:
just search `XKB_LOG_MESSAGE_NO_ID`.