adduser/addgroup: check username for invalid chars

(by Tito <farmatito AT tiscali.it>). +129 bytes when enabled.

libbb/Kbuild

@@ -122,6 +122,7 @@ lib-$(CONFIG_MKFS_MINIX) += find_mount_point.o
 lib-$(CONFIG_SELINUX) += selinux_common.o
 lib-$(CONFIG_HWCLOCK) += rtc.o
 lib-$(CONFIG_RTCWAKE) += rtc.o
+lib-$(CONFIG_FEATURE_CHECK_NAMES) += die_if_bad_username.o
 
 # We shouldn't build xregcomp.c if we don't need it - this ensures we don't
 # require regex.h to be in the include dir even if we don't need it thereby