Rename safe_strncpy() to strlcpy().

Rename safe_strncpy() to strlcpy().

This cleans up the use of safe_strncpy() even more.  Since it has the
same semantics as strlcpy() use this name instead.  Also move the
definition from inside path.c to its own file compat/strlcpy.c, and use
it conditionally at compile time, since some platforms already has
strlcpy().  It's included in the same way as compat/setenv.c.

Signed-off-by: Peter Eriksen <s022018@student.dtu.dk>
Signed-off-by: Junio C Hamano <junkio@cox.net>