libutil++: Rename manpage source files to avoid colons

To permit checking the sources out on systems such as Windows with
more restrictive file name requirements, rename the manpage source
files to replace colons with underscrores.  Use MANSRC.foo to point
<bsd.man.mk> at the new source file names.

Reviewed by:	ivy
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D51794

lib/libutil++/Makefile

@@ -11,6 +11,10 @@ MAN+=	freebsd::FILE_up.3 \
 	freebsd::pidfile.3 \
 	freebsd::stringf.3
 
+.for page in ${MAN}
+MANSRC.${page}=	${page:S/:/_/g}
+.endfor
+
 .include <src.opts.mk>
 
 HAS_TESTS=