Separate multi-words filenames by '_'

Rename foobar.ext to foo_bar.ext.

<https://github.com/Genymobile/scrcpy/pull/226#discussion_r209454865>

app/src/str_util.c

@@ -0,0 +1,33 @@
+#include "str_util.h"
+
+size_t xstrncpy(char *dest, const char *src, size_t n) {
+    size_t i;
+    for (i = 0; i < n - 1 && src[i] != '\0'; ++i)
+        dest[i] = src[i];
+    if (n)
+        dest[i] = '\0';
+    return src[i] == '\0' ? i : n;
+}
+
+size_t xstrjoin(char *dst, const char *const tokens[], char sep, size_t n) {
+    const char *const *remaining = tokens;
+    const char *token = *remaining++;
+    size_t i = 0;
+    while (token) {
+        if (i) {
+            dst[i++] = sep;
+            if (i == n)
+                goto truncated;
+        }
+        size_t w = xstrncpy(dst + i, token, n - i);
+        if (w >= n - i)
+            goto truncated;
+        i += w;
+        token = *remaining++;
+    }
+    return i;
+
+truncated:
+    dst[n - 1] = '\0';
+    return n;
+}