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 af782b9f2ac4fb817ded80d4317a45345bb3f992)

Change-Id: Ic76ffc9412171dddc879af0bbf6e20fbe1a8f057
diff --git a/adb/adb_auth.h b/adb/adb_auth.h
index b24c674..54dd537 100644
--- a/adb/adb_auth.h
+++ b/adb/adb_auth.h
@@ -18,6 +18,7 @@
 #define __ADB_AUTH_H
 
 void adb_auth_init(void);
+int adb_auth_keygen(const char* filename);
 void adb_auth_verified(atransport *t);
 
 void send_auth_request(atransport *t);