MFC r261774 by feld:

	Add caveat to zpool manpage indicating that we do not automatically activate
	hot spares. This should be MFC'd to all STABLE branches.

	Upon the availability of zfsd, the zpool manpage on relevant branches should
	be updated to remove this caveat and document hot spare's reliance on zfsd.

Requested by:	feld
MFC: r261773

In acline_init(), initialize ac_line to SRC_UNKNOWN.  Previously
this could lead to the -n option effectively being ignored (in case
ac_line happened to be 0 aka SRC_AC), or other undefined behaviour.

PR:		169779
Submitted by:	Alex Gonzalez <loox at e-shell.com>
Reviewed by:	jhb
MFC: r262296

Match the correct variable to the variable description.

PR:		121173
Submitted by:	Thomas Mueller <tmueller at sysgo.com>
MFC: r262574

Add tws(4) to the hardware notes.