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/shared/bus-unit-util.c

@@ -1639,7 +1639,6 @@ static int bus_append_root_hash(sd_bus_message *m, const char *field, const char
 }
 
 static int bus_append_root_hash_signature(sd_bus_message *m, const char *field, const char *eq) {
-        char *value;
         _cleanup_free_ void *roothash_sig_decoded = NULL;
         size_t roothash_sig_decoded_size = 0;
         int r;
@@ -1648,7 +1647,8 @@ static int bus_append_root_hash_signature(sd_bus_message *m, const char *field,
         if (path_is_absolute(eq))
                 return bus_append_string(m, "RootHashSignaturePath", eq);
 
-        if (!(value = startswith(eq, "base64:")))
+        const char *value = startswith(eq, "base64:");
+        if (!value)
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                        "Failed to decode %s value '%s': neither a path nor starts with 'base64:'.",
                                        field, eq);

src/shared/pager.c

@@ -298,7 +298,7 @@ bool pager_have(void) {
 
 int show_man_page(const char *desc, bool null_stdio) {
         const char *args[4] = { "man", NULL, NULL, NULL };
-        char *e = NULL;
+        const char *e = NULL;
         pid_t pid;
         size_t k;
         int r;

src/shared/seccomp-util.c

@@ -2505,7 +2505,7 @@ uint32_t scmp_act_kill_process(void) {
 
 int parse_syscall_and_errno(const char *in, char **name, int *error) {
         _cleanup_free_ char *n = NULL;
-        char *p;
+        const char *p;
         int e = -1;
 
         assert(in);

src/shared/vpick.c

@@ -343,7 +343,7 @@ static int make_choice(
                 unsigned found_tries_done = UINT_MAX, found_tries_left = UINT_MAX;
                 _cleanup_free_ char *dname = NULL;
                 size_t found_architecture_index = SIZE_MAX;
-                const char *e;
+                char *e;
 
                 dname = strdup((*entry)->d_name);
                 if (!dname)