Use const_strlen in a few different places

This may slightly improve performance by allowing the compiler greater
visibility into what is happing on top of not executing at runtime in
some hot paths, but more importantly, it gets rid of magic constants in a
few different places.

src/postfork.cpp

@@ -425,9 +425,9 @@ static char *get_interpreter(const char *command, char *buffer, size_t buff_size
         close(fd);
     }
 
-    if (std::strncmp(buffer, "#! /", 4) == 0) {
+    if (std::strncmp(buffer, "#! /", const_strlen("#! /")) == 0) {
         return buffer + 3;
-    } else if (std::strncmp(buffer, "#!/", 3) == 0) {
+    } else if (std::strncmp(buffer, "#!/", const_strlen("#!/")) == 0) {
         return buffer + 2;
     }
     return nullptr;