kmk: Add better support for generating config.h and confdefs.h headers.

This includes the `config_headers` unit and the CONFIG_HEADERS targets.
kmk: Generate mozilla-config.h using CONFIG_HEADERS.
[OS/2] Update mozilla-config.os2.h with more up-to-date values.
kmk: Make search pattern more strict in config_headers unit.

This is to avoid partial matches of macros in template files.

Also fix a silly typo.
kmk: Generate xpcom-config.h and xpcom-private.h using CONFIG_HEADERS.
kmk: Install global (config) headers.
kmk: Fix creating output directories in `config_headers` unit.
kmk: nspr: Fix sanity checks and output directory creation.
kmk: Brush up target templates.