According to devctl(4), clients must read events whole; they may not
piece them together from multiple reads().  It's as if /dev/devctl is
a datagram device instead of a stream device.  However, devd's
internal buffer was too small (1025 bytes) to read an entire
ereport.fs.zfs.checksum event (variable, up to ~1300 bytes).  This
commit enlarges the buffer to 8k.

Reviewed by:	imp
Approved by:	ken (mentor)
MFC after:	2 weeks
Fix examples for overriding INSTALL to not suggest hardcoding
'install' since it breaks buildworld after the introduction and
use of 'install -l' in r245752. Overriding INSTALL causes
/usr/bin/install to be used instead of the proper
/usr/src/tools/install.sh which handles the new flag.

Approved by:	bapt
MFC after:	2 weeks