libntp/authreadkeys.c uses banned function strncpy()

Per https://gitlab.com/NTPsec/ntpsec/blob/master/devel/hacking.adoc:

The following C99/POSIX functions are BANNED.  They are unsafe, tending to
cause buffer overruns and (all too often) exploitable security holes:

* strcpy, strncpy, strcat:  Use strlcpy and strlcat instead.

These function calls were added to libntp/authreadkeys.c in d4691e60 ("Add CMAC authentication")

/cc @hal.murray