Delete more rtc_base/stringutils.*

Delete nonnull, strchr, strchrn, strcatn, strlenn and Traits.

Bug: webrtc:6424
Change-Id: I3b5a48cb71c6de33635f25ef64d941c422ad0881
Reviewed-on: https://webrtc-review.googlesource.com/c/106341
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25232}

rtc_base/stringutils.cc

@@ -14,6 +14,24 @@
 
 namespace rtc {
 
+size_t strcpyn(char* buffer,
+               size_t buflen,
+               const char* source,
+               size_t srclen /* = SIZE_UNKNOWN */) {
+  if (buflen <= 0)
+    return 0;
+
+  if (srclen == SIZE_UNKNOWN) {
+    srclen = strlen(source);
+  }
+  if (srclen >= buflen) {
+    srclen = buflen - 1;
+  }
+  memcpy(buffer, source, srclen);
+  buffer[srclen] = 0;
+  return srclen;
+}
+
 void replace_substrs(const char* search,
                      size_t search_len,
                      const char* replace,