always check valid debug level is specified

debug level value is only checked when applied locally additionally, an invalid debug level still incorrectly displays a success message in the console (but will display the error in the log file)

EOS Console [root://localhost] |/> debug warn /eos/*/fst
success: switched to mgm.debuglevel=warn on nodes mgm.nodename=/eos/*/fst

EOS Console [root://localhost] |/> debug warn /eos/*/fst
success: switched to mgm.debuglevel=warn on nodes mgm.nodename=/eos/*/fst
success: switched to mgm.debuglevel=warn on nodes mgm.nodename=/eos/*/mgm
error: debug level warn is not known! (errc=22) (Invalid argument)

EOS Console [root://localhost] |/> debug warn
error: debug level warn is not known! (errc=22) (Invalid argument)

modified to always check before sending:

EOS Console [root://localhost] |/> debug cats
error: debug level cats is not known! (errc=22) (Invalid argument)
EOS Console [root://localhost] |/> debug cats *
error: debug level cats is not known! (errc=22) (Invalid argument)
EOS Console [root://localhost] |/> debug cats /eos/*/fst
error: debug level cats is not known! (errc=22) (Invalid argument)

common/RWMutex.cc

@@ -139,10 +139,10 @@ RWMutex::RWMutex(bool prefer_rd):
   ResetTimingStatistics();
 #endif
 
-  if (getenv("EOS_USE_SHARED_MUTEX")) {
-    mMutexImpl = new SharedMutex();
-  } else {
+  if (getenv("EOS_USE_PTHREAD_MUTEX")) {
     mMutexImpl = new PthreadRWMutex(prefer_rd);
+  } else {
+    mMutexImpl = new SharedMutex();
   }
 }