This commit clarifies some things inside WriteCharsLegacy by adding
comments and renaming parameter and enum names. It does not change any
logic.
`WC_ECHO` was used extensively to mean only one thing: whether to print
a control character like `\x18` (Ctrl+X>) as `^X`. It's been renamed
to make that abundantly clear.
Many include statements use forward slashes, while others use backwards
slashes. This is inconsistent formatting. For this reason, I changed the
backward slashes to forward slashes since that is the standard.
