internal/app: less strict username regex

Use the default value of NAME_REGEX from adduser. Should not hurt compatibility while being less strict.

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

internal/app/username.go

@@ -0,0 +1,12 @@
+package app
+
+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)
+}