utils: strlcpy() and strlcat() don't clobber dst

utils: strlcpy() and strlcat() don't clobber dst

As David Laight correctly pointed out, the first version of strlcpy()
modified dst buffer behind the string copied into it. Fix this by
writing NUL to the byte immediately following src string instead of to
the last byte in dst. Doing so also allows to reduce overhead by using
memcpy().

Improve strlcat() by avoiding the call to strlcpy() if dst string is
already full, not just as sanity check.

Signed-off-by: Phil Sutter <phil@nwl.cc>