Perform configuration checks in each init script

Some checks have been removed:
- xrootd init script doesn't check for mysqld socket
  file as xrootd error message is clear enough.
- MySQL init script doesn't check for unix user as it is not
  used for now in init.d script. Indeed user option is used
  by mysqlmanager which is not available in MySQL version
  provided by eups.
Use functions for initscripts configuration checks

Main configuration checks are now factorized in
check_readable() and check_writable().