Added multiple security tests to pgp keyring against disk full and exceeded quota.

- copy files to tmp before appending new keys, then rename back to original.
- always check for disk full before syncing keyrings
- use tmp file to sync keyring
- new RsDirUtil::fileExists() method
- check system for disk space in pgp directory
- added RsPGPDirectory() method to RsInit


git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@6541 b45a01b8-16f6-495d-af2f-9b41ad6348cc

libretroshare/src/retroshare/rsinit.h

@@ -126,6 +126,7 @@ class RsInit
 		 */
 		static std::string 	RsConfigDirectory();
 		static std::string 	RsConfigKeysDirectory();
+		static std::string 	RsPGPDirectory();
 
 		static std::string  RsProfileConfigDirectory();
 		static bool         getStartMinimised() ;