Rename ZipEntryName to ZipString

since the struct is now used for other purposes. Also add some
comparator functions to the struct to simplify zip_archive.cc.

This is a follow-up CL for f1d3d3b2477a813805b71099c60e07d36dbb225a.

Bug: 21957428
Change-Id: I60d4171eeacc561d59226d946e9eb5f9c96d80cf
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 4c4c95b..b964a36 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -391,7 +391,7 @@
 
 static void* unzip_file(ZipArchiveHandle zip, const char* entry_name, unsigned* sz)
 {
-    ZipEntryName zip_entry_name(entry_name);
+    ZipString zip_entry_name(entry_name);
     ZipEntry zip_entry;
     if (FindEntry(zip, zip_entry_name, &zip_entry) != 0) {
         fprintf(stderr, "archive does not contain '%s'\n", entry_name);
@@ -453,7 +453,7 @@
         return -1;
     }
 
-    ZipEntryName zip_entry_name(entry_name);
+    ZipString zip_entry_name(entry_name);
     ZipEntry zip_entry;
     if (FindEntry(zip, zip_entry_name, &zip_entry) != 0) {
         fprintf(stderr, "archive does not contain '%s'\n", entry_name);