tree-wide: Fix constness issues with newer glibc (#39896)

Latest glibc uses _Generic to have strstr() and other functions return
const char* or char* based on whether the input is a const char* or a
char*. This causes build failures as we previously always expected a
char*.

Let's fix the compilation failures and add our own macros similar to
glibc's to have string functions that return a mutable or const pointer
depending on the input.

src/basic/time-util.c

@@ -1156,9 +1156,7 @@ static const char* extract_multiplier(const char *p, usec_t *ret) {
         assert(ret);
 
         FOREACH_ELEMENT(i, table) {
-                char *e;
-
-                e = startswith(p, i->suffix);
+                const char *e = startswith(p, i->suffix);
                 if (e) {
                         *ret = i->usec;
                         return e;
@@ -1334,9 +1332,7 @@ static const char* extract_nsec_multiplier(const char *p, nsec_t *ret) {
         assert(ret);
 
         FOREACH_ELEMENT(i, table) {
-                char *e;
-
-                e = startswith(p, i->suffix);
+                const char *e = startswith(p, i->suffix);
                 if (e) {
                         *ret = i->nsec;
                         return e;