size_t strnlen_PF(uint_farptr_t src, size_t len)
The strnlen_PF() function is similar to strnlen(), except that s is a far pointer to a string in program space
The strnlen_PF function returns strlen_P(s), if that is less than len, or len if there is no '\0' character among the first len characters pointed to by s. The contents of RAMPZ SFR are undefined when the function returns