Update irodsServerMonPerf

solution for "top: failed tty get" on some linux (CentOS 6 in my case) 
On some systems top will not tolerate not being attached to a tty
-b invokes batch mode a non-interactive mode for sending output to other programs or a file
Untested on other platforms
Update irodsServerMonPerf

Added -P flag to df calls 
...
       -P, --portability
              use the POSIX output format
...
solves a parsing issue on CentOS6 

...
-bash-4.1$ LANG=C;export LANG;df -k /
Filesystem           1K-blocks    Used Available Use% Mounted on
/dev/mapper/sys-sysroot
                      20642428 1849748  17744104  10% /
...
...
-bash-4.1$ LANG=C;export LANG;df -kP /
Filesystem              1024-blocks    Used Available Capacity Mounted on
/dev/mapper/sys-sysroot    20642428 1849748  17744104      10% /
...

Note: if you have run this and it has created the configuration file ( /var/lib/irods/iRODS/server/config/scriptMonPerf.config ), it will continue to use the wrong column until that file is removed and regenerated with the next run of the script.
Merge pull request #2079 from SyBernot/master

Update irodsServerMonPerf