feat(linux): comments for rework keymap (second PR)🐘

[SabineSIL]

fixes parts of PR https://github.com/keymanapp/keyman/pull/9384. Some comments have been moved into an issue https://github.com/keymanapp/keyman/issues/11802

This PR addresses and bundles comments made on https://github.com/keymanapp/keyman/pull/9384 for keymap (second PR)

• remove "language","us" as parameter
• openFile
• leak fp
• layout
• str_buf variable
• layout
• include comment
• counting of index
• avoid casting

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

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