man: encode minushyphen the way groff/man requires it

man: encode minushyphen the way groff/man requires it

Sparked by a recent LWN article[1], sweeps over the iptables manpages
for incorrectly encoded dashes was made by Phil Sutter and myself.

An ASCII minushyphen in the source manpage translates to a hyphen in
output, so one has to use the sequence "\-" to get a minushyphen in
the output, as groff_char(7) explains.

[1] https://lwn.net/Articles/947941/ (paywalled until about 2023-11-06)

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>