errno-util: return const string from strerror_safe()

errno-util: return const string from strerror_safe()

C's strerror() function does not return a "const char *" pointer
for the string. That has historic reasons and C99 even comments
that "[t]he array pointed to shall not be modified by the program".

Make the strerror_safe() wrapper correct this and be more strict
in this regard.