feat(linux): comments for mc_kmxfile 🐘

[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 the module mc_kmxfile.

• use u16string instead of PKMX_WCHAR
• change *2 + 2
• change for -loop
• change for -loop again
• check that all bytes were written
• use char* for filename
• sz<=0
• rename sz
• delete buf 1
• delete buf 2
• delete buf 3
• delete buf 4
• edit cast
• check p==NULL
• move KMX_WriteCompiledKeyboard
• initialize lpKeyboard

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[keymanapp-test-bot[bot]]

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