internal/validate: relocate from app

These are free of the dispatcher from internal/app. This change relocates them into their own package.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/validate/username.go

@@ -0,0 +1,12 @@
+package validate
+
+import "regexp"
+
+// nameRegex is the default NAME_REGEX value from adduser.
+var nameRegex = regexp.MustCompilePOSIX(`^[a-zA-Z][a-zA-Z0-9_-]*\$?$`)
+
+// IsValidUsername returns whether the argument is a valid username.
+func IsValidUsername(username string) bool {
+	return len(username) < Sysconf(SC_LOGIN_NAME_MAX) &&
+		nameRegex.MatchString(username)
+}