strings.h is the header mandated for these functions in POSIX.1 prior to
2008 (when most of these were removed). The declarations in string.h are
only for BSD compatibility. But when both headers are included, avoid
duplicate declarations.
Also, mark stpcpy and stpncpy as POSIX.1-2008.
Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>