New possibility of "dry run"

If nstep=0, pw.x performs a quick "dry run" with fast initializations and
nothing else. Simpler alternative to starting with a "prefix".EXIT file
present in the scratch directory. The nstep=0 case was not usable anyway.

PW/Doc/INPUT_PW.def

@@ -179,7 +179,9 @@ input_description -distribution {Quantum Espresso} -package PWscf -program pw.x
         var nstep -type INTEGER {
             info {
                 number of molecular-dynamics or structural optimization steps 
-                performed in this run
+                performed in this run. If set to 0, the code performs a quick
+                "dry run", stopping just after initialization. This is useful
+                to check for input correctness and to have the summary printed.
             }
             default {
                 1  if @ref calculation == 'scf', 'nscf', 'bands';

PW/Doc/user_guide.tex

@@ -340,7 +340,9 @@ the working directory  is the directory where the executable is!
 The advantage of this procedure is that all files are properly closed, 
 whereas  just killing the process may leave data and output files in 
 an unusable state. If you start the execution with the EXIT file already
-in place, the code will stop after initialization. This is useful to have a quick check of the correctness of the input.
+in place, the code will stop after initialization. Alternatively:
+set \texttt{nstep} to 0 in input. This is useful to have
+a quick check of the correctness of the input. 
 
 \subsection{Electronic structure calculations}
 \paragraph{Single-point (fixed-ion) SCF calculation} 

PW/src/run_pwscf.f90

@@ -109,7 +109,7 @@ SUBROUTINE run_pwscf( exit_status )
   ! ... dry run: code will stop here if called with exit file present
   ! ... useful for a quick and automated way to check input data
   !
-  IF ( check_stop_now() ) THEN
+  IF ( nstep == 0 .OR. check_stop_now() ) THEN
      CALL pre_init()
      CALL data_structure( gamma_only )
      CALL summary()