doc/*.6: Tweak copyright notices in man pages

Use *roff special character `\(co` for copyright sign.  This special
character identifier is _almost_ universally portable, dating back to
Ossanna troff in 1976.[1]  The exception is Plan 9 nroff, which claims
to be thoroughly UTF-8 oriented and should be able to render a copyright
sign.[2]  If no glyph for the character is available, the output device
driver degrades it to "c" (which is technically correct[3]), except for
groff, which falls back to "(C)"[4] only on its "ascii" output device.
All of groff's other devices have a a proper copyright sign.[5]

References:
[1] https://archive.org/details/unix-programmers-manual-seventh-edition-vol-2-1983/
    (pp. 196ff., see esp. "Table II", p. 227)
[2] https://github.com/9fans/plan9port/pull/735#issuecomment-3391733330
[3] https://www.copyright.gov/circs/circ03.pdf
[4] https://github.com/GitMirroring/groff/blob/a4d6e619afc8042b12084527c3c11eae919533e2/tmac/tty.tmac#L123
[5] https://paste.c-net.org/ComradesCourage

doc/recover.6

@@ -148,5 +148,11 @@ determine.
 should be taught to use the nethack playground locking mechanism to
 avoid conflicts.
 .SH COPYRIGHT
-This file is Copyright (C) \*(Na, \*(Nd for version \*(Nb:\*(Nr.
+.\" All troffs and nroffs support `\(co` except Plan 9 nroff, but Plan 9
+.\" offers no test for special character availability, and its feature
+.\" set too closely resembles DWB 3.3 to test for it.  Use a string with
+.\" any nroff-mode formatter not claiming groff compatiblity.
+.ds co \(co
+.if !\n(.g .if n .ds co (C)\" in principle, should be just "c"
+This file is Copyright \*(co \*(Na, \*(Nd for version \*(Nb:\*(Nr.
 NetHack may be freely redistributed.  See license for details.