doxygen: Improve DOXYGEN_defines comment
doxygen: Get rid of DOXYGEN_tagbase
system: Make DirExists() return the path if it exists
system: Add FileExists()
make: Add Dir() and NotDir() that handle paths with spaces
system: Round out FindFile(), FindFileI(), FindDir(), FindDirI() functions
system: $(if)'s third parameter is optional
system: Escape pattern characters in ShellEscape()
system: FindFiles() -> FindFilesUnder()
system: No need for separate Find*() functions for case [in]sensitive

Now they're case-sensitive except on Windows (just like almost everything else)
system: Handle multiple Program Files directories on 64-bit Windows

- Add SYSTEM_PROGRAMFILESx86
- Add SYSTEM_FindProgramFilesFile() for finding files in both
- Add SYSTEM_FindProgramFilesDir() for finding directories in both
system: Correct tokens in SYSTEM_SHELL_ENCODEWORD
system: Fix typos in FindFiles() and FindDirs()