check_dhcp: fix tests for machines with default interace != eth0
check_http: tests use supplied value if possible
check_procs: fix test with sleeping processes

at least on freebsd, there are a processes with state 'S', they always
use two character status.
check_disk_smb: verify path to smbclient
check_disk: get_fs_usage hasn't been run if using groups

the reason why it still worked sometimes was fsu_blocks beeing uninitialized
which resulted in a true test sometimes.