Support SNI in libfetch

SNI is Server Name Indentification which is a protocol for TLS that
indicates the host that is being connected to at the start of the
handshake. It allows to use Virtual Hosts on HTTPS.

Submitted by:	sbz
Submitted by:	Michael Gmelin <freebsd@grem.de> [1]
PR:		kern/183583 [1]
Reviewed by:	des
Approved by:	bapt
MFC after:	1 week
Follow-up to r258227 and document 'enabled' as a boolean instead of
a string.

Approved by:	bapt
MFC after:	2 days
X-MFC-With:	r258227