Implemented strlcat, stpncpy, mem(r)mem, and strchrnul

[ZERICO2005]

Implements other common POSIX/GNU/BSD functions: strlcat, stpncpy, memmem, memrmem, strrstr, and strchrnul.

This implementation of memmem has a time complexity of O(N^2) although O(N) algorithms do exist. The worst cases only arise when looking for "az" in "abacadaeafag...". In general, the maximum calls to memcmp is proportional to the number of times that the first character of needle (in this case a) appears in haystack.

stpncpy is faster than strncpy at zero padding dst. I implemented the zero padding with lddr (1R + 1W + 1 per \0) whereas strncpy uses a 6F + 1R + 1W + 1 per \0 loop https://github.com/pcawte/AgDev/blob/main/src/libc/strncpy.src