Various fixes for ggatec and ggated

Dynamically size buffers in ggatec. Instead of static size on the stack.
Add flush support.

Submitted by: Johannes Totz <jo@bruelltuete.com>
Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D31722

(cherry picked from commit 6226477a462f5ffbeacafdc9461524c95a7eb154)

geom_gate: ensure readprov is null-terminated

With crafted input to the G_GATE_CMD_CREATE ioctl, geom_gate can be made
to print kernel memory to the system console, potentially revealing
sensitive data from whatever was previously in that memory page.

But but but: this is a case of the sys admin misconfiguring, and you'd
need root privileges to do this.

Submitted By: Johannes Totz <jo@bruelltuete.com>
Reviewed By: asomers
Differential Revision: https://reviews.freebsd.org/D31727

(cherry picked from commit f284bed200b04e48c4ae87a50f4a8a957b0a10ad)