strv: declare iterator of FOREACH_STRING() in the loop

Same idea as 03677889f0.

No functional change intended. The type of the iterator is generally changed to
be 'const char*' instead of 'char*'. Despite the type commonly used, modifying
the string was not allowed.

I adjusted the naming of some short variables for clarity and reduced the scope
of some variable declarations in code that was being touched anyway.

src/basic/fs-util.c

@@ -570,7 +570,6 @@ int get_files_in_directory(const char *path, char ***list) {
 }
 
 static int getenv_tmp_dir(const char **ret_path) {
-        const char *n;
         int r, ret = 0;
 
         assert(ret_path);

src/basic/os-util.c

@@ -170,8 +170,6 @@ int open_extension_release(const char *root, const char *extension, char **ret_p
                         }
                 }
         } else {
-                const char *p;
-
                 FOREACH_STRING(p, "/etc/os-release", "/usr/lib/os-release") {
                         r = chase_symlinks(p, root, CHASE_PREFIX_ROOT,
                                            ret_path ? &q : NULL,

src/basic/strv.h

@@ -218,7 +218,7 @@ void strv_print(char * const *l);
         })
 
 #define _FOREACH_STRING(uniq, x, y, ...)                                \
-        for (char **UNIQ_T(l, uniq) = STRV_MAKE(({ x = y; }), ##__VA_ARGS__); \
+        for (const char *x, * const*UNIQ_T(l, uniq) = STRV_MAKE_CONST(({ x = y; }), ##__VA_ARGS__); \
              x;                                                         \
              x = *(++UNIQ_T(l, uniq)))
 

src/basic/unit-file.c

@@ -236,7 +236,6 @@ bool lookup_paths_timestamp_hash_same(const LookupPaths *lp, uint64_t timestamp_
 }
 
 static int directory_name_is_valid(const char *name) {
-        const char *suffix;
 
         /* Accept a directory whose name is a valid unit file name ending in .wants/, .requires/ or .d/ */