improve nxagent manpage

[uli42]

• option -co is no longer available
• option -verbose is not documented
• option -keystrokefile is not documented
• description of SIGHUP handling is wrong:

.I SIGHUP
This signal causes the server to close all existing connections, free all
resources, and restore all defaults.  It is sent by the display manager
whenever the main user's main application (usually an \fIxterm\fP or window
manager) exits to force the server to clean up and prepare for the next
user.

Probably more...

[uli42]

-co has already be removed from the manpage, the rest is still valid

[uli42]

-extensions is not documented (it is a nop, but checked by Args.c, so it should either be removed or documented)