Introduce "adb keygen"

Introduce the "adb keygen" command.

  Usage: adb keygen <filename>

This command creates an adb public/private key pair in a user
specified file. This can be used to create new adb keys, or rotate
existing keys.

Modify adb's key generation routines to use the HOSTNAME/LOGNAME
environment variables if available. This allows someone to override
the username/hostname embedded within the adb public key file if
desired. Fallback to the old mechanisms if those environment
variables aren't available.

Bug: 18342715
Change-Id: Ibccee6088d4609aa05ad6687d3a1d8a8689d3e8a

(cherry picked from commit a85094151253845648ee005325dcd0a254270253)

Change-Id: Ic76ffc9412171dddc879af0bbf6e20fbe1a8f057
diff --git a/adb_auth_host.c b/adb_auth_host.c
index c72fe42..4c33d14 100644
--- a/adb_auth_host.c
+++ b/adb_auth_host.c
@@ -114,18 +114,34 @@
 static void get_user_info(char *buf, size_t len)
 {
     char hostname[1024], username[1024];
-    int ret;
+    int ret = -1;
+
+    if (getenv("HOSTNAME") != NULL) {
+        strncpy(hostname, getenv("HOSTNAME"), sizeof(hostname));
+        hostname[sizeof(hostname)-1] = '\0';
+        ret = 0;
+    }
 
 #ifndef _WIN32
-    ret = gethostname(hostname, sizeof(hostname));
     if (ret < 0)
+        ret = gethostname(hostname, sizeof(hostname));
 #endif
+    if (ret < 0)
         strcpy(hostname, "unknown");
 
+    ret = -1;
+
+    if (getenv("LOGNAME") != NULL) {
+        strncpy(username, getenv("LOGNAME"), sizeof(username));
+        username[sizeof(username)-1] = '\0';
+        ret = 0;
+    }
+
 #if !defined _WIN32 && !defined ADB_HOST_ON_TARGET
-    ret = getlogin_r(username, sizeof(username));
     if (ret < 0)
+        ret = getlogin_r(username, sizeof(username));
 #endif
+    if (ret < 0)
         strcpy(username, "unknown");
 
     ret = snprintf(buf, len, " %s@%s", username, hostname);
@@ -436,6 +452,11 @@
     return ret + 1;
 }
 
+int adb_auth_keygen(const char* filename) {
+    adb_trace_mask |= (1 << TRACE_AUTH);
+    return (generate_key(filename) == 0);
+}
+
 void adb_auth_init(void)
 {
     int ret;