mark functions and vars as static (#146)

the goal here to mark functions and variables not used outside the
translation unit as static. main reason for this is cleanliness. however
as a side-effect this can help compilers optimize better as it now has
guarantee that a certain function won't be called outside of that
translation unit.

one other side-effect of this is that accessing these vars/function from
config.h is now different.

if one wants to access a static var/func from different translation unit
in config.h, he would have to create a wrapper function under the right
ifdef. for static functions one would also need to forward declare it.
here's a dummy example of accessing the function `run_key_handler` from
config.h under _MAPPINGS_CONFIG

```
static void run_key_handler(const char *, unsigned);
bool send_with_ctrl(arg_t key) {
	run_key_handler(XKeysymToString(key), ControlMask);
	return false;
}
```

options.c

@@ -26,8 +26,7 @@
 #include <string.h>
 #include <unistd.h>
 
-opt_t _options;
-const opt_t *options = (const opt_t*) &_options;
+const opt_t *options;
 
 void print_usage(void)
 {
@@ -36,7 +35,7 @@ void print_usage(void)
 	       "[-z ZOOM] FILES...\n");
 }
 
-void print_version(void)
+static void print_version(void)
 {
 	puts("nsxiv " VERSION);
 }
@@ -46,6 +45,8 @@ void parse_options(int argc, char **argv)
 	int n, opt;
 	char *end, *s;
 	const char *scalemodes = "dfFwh";
+	static opt_t _options;
+	options = &_options;
 
 	progname = strrchr(argv[0], '/');
 	progname = progname ? progname + 1 : argv[0];