app: check username length against LOGIN_NAME_MAX

This limit is arbitrary, but it's good to enforce it anyway.

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

internal/sysconf.go

@@ -0,0 +1,6 @@
+package internal
+
+//#include <unistd.h>
+import "C"
+
+func Sysconf_SC_LOGIN_NAME_MAX() int { return int(C.sysconf(C._SC_LOGIN_NAME_MAX)) }