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.

src/basic/errno-util.h

@@ -31,7 +31,7 @@ static inline int negative_errno(void) {
         return -errno;
 }
 
-static inline char *strerror_safe(int error) {
+static inline const char *strerror_safe(int error) {
         /* 'safe' here does NOT mean thread safety. */
         return strerror(abs(error));
 }