- Part 2, add @@hostname system variable


mysql-test/r/variables.result:
  Add testcase for @@hostname
mysql-test/t/variables.test:
  Add testcase for @@hostname
sql/log.cc:
  Use pidfile_name as base when generating the default log name
sql/mysqld.cc:
  Init "glob_hostname" to that of hostname, fallback to "localhost"
  if 'gethostname' should fail.
  Init "pidfile_name" to hostname.pid and fallback to "mysql.pid" if
  'gethostname' should fail
sql/set_var.cc:
  Add new system variable "hostname"