bhyvectl: reduce code duplication

Combine send_start_checkpoint() and send_start_suspend() into a
single function named snapshot_request().

snapshot_request() is equivalent to send_start_checkpoint() and
send_start_suspend() except that it takes an additional argument. The
additional argument, enum ipc_opcode, is used to determine the type of
snapshot request being performed. Also, switch to using strlcpy instead
of strncpy.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D28878