com: replace BSD strlcpy and strlcat

com: replace BSD strlcpy and strlcat

Compared to BSD strlcpy and strlcat, dp_strlcpy and dp_strlcat are
faster, never crash, and have a more useful return (DP didn't use the
strlcpy return at all), see included docs.

Adds dp_stpecpy() for efficient chain copying and dp_ustr2stp() for
copying measured byte sequences (unterminated strings) to strings.

Replaces the only use of the strlcat() return with dp_stpecpy().

Updates the forbidden string functions list.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>