Allow vt(4) to disable terminal bell with `sysctl kern.vt.bell_enable=0`,
similar as syscons(4) do.

Submitted by:	Tiwei Bie <btw@mail.ustc.edu.cn>
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Support the Vodafone R215 LET USB dongle, which is apparently a rebadged
E5372 with different product IDs.

Interestingly, the standard E5372 IDs (12d1:1506) are currently listed in
u3g.c and are the same as the E3131.  However, the R215/E5372 is an NCM
device and works well with cdce(4) whereas the E3131 isn't.  More work
may be needed to better identify the other device IDs.

MFC after:	1 week
Remove call to access(2) which didn't serve any purpose, and make it more
tolerant to errors.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation