Import spl_hostid as a module parameter.

Provide a call_usermodehelper() alternative by letting the hostid be passed as
a module parameter like this:

$ modprobe spl spl_hostid=0x12345678

Internally change the spl_hostid variable to unsigned long because that is the
type that the coreutils /usr/bin/hostid returns.

Move the hostid command into GET_HOSTID_CMD for consistency with the similar
GET_KALLSYMS_ADDR_CMD invocation.

Use argv[0] instead of sh_path for consistency internally and with other Linux
drivers.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>