size_t strlcat_P(char *, const char *, size_t)
The strlcat_P() function is similar to strlcat(), except that the src string must be located in program space (flash).
Appends src to string dst of size siz (unlike strncat(), siz is the full size of dst, not space left). At most siz-1 characters will be copied. Always NULL terminates (unless siz <= strlen(dst)).
The strlcat_P() function returns strlen(src) + MIN(siz, strlen(initial dst)). If retval >= siz, truncation occurred.