merge: Avoid using c++ reserved identifiers

 * Replace `try` variable with `user`
 * Replace `virtual` variable with `vnum`
 * Replace `template` variable with `tmpl`
 * Replace `catch` variable with `restore`
 * Replace `new` variable with `has_new_mail`
 * Replace `new` variable with`new_file`
 * Replace `public` variable with `only_public_key`
 * Replace `delete` variable with `delete_original`
 * Replace `new` variable with `mark_new`
 * Replace `new` variable with `al_new`
 * Replace `new` variable with `alias`
 * Replace `new` variable with `ap`
 * Replace `new` with `pnew`
 * Unify `body_new` name instances
 * Replace `new` variable with `add`
 * Replace `new` variable with `new_mail`
 * Replace `new` variable with `new_tag`
 * Replace `new` variable with `new_label`
 * Replace `new` variable with `tnew`
 * Replace `new` variable with `new_mail`
 * Replace `delete` variable with `delete_header`
 * Replace `this` variable with `ctx_cur`
 * Replace `this` variable with `key`
 * Replace `class` variable with `qc`
 * Replace `new` variable with `new_param`
 * Replace `new` variable with `new_label`
 * Replace `class` member with `flags`
 * Replace `not` member with `pat_not`
 * Replace `or` member with `pat_or`
 * Fix functions declarations/definitions