tree-wide: use STRLEN() to allocate buffer of constant size

Using strlen() to declare a buffer results in a variable-length array,
even if the compiler likely optimizes it to be a compile time constant.

When building with -Wvla, certain versions of gcc complain about such
buffers. Compiling with -Wvla has the advantage of preventing variably
length array, which defeat static asserts that are implemented by
declaring an array of negative length.

src/basic/fd-util.c

@@ -368,7 +368,7 @@ bool fdname_is_valid(const char *s) {
 }
 
 int fd_get_path(int fd, char **ret) {
-        char procfs_path[strlen("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
+        char procfs_path[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
         int r;
 
         xsprintf(procfs_path, "/proc/self/fd/%i", fd);