groff 1.23.0 added .MR to its -man macro package.  The NEWS file states
that the inclusion of the macro "was prompted by its introduction to
Plan 9 from User Space's troff in August 2020."  From d32deab it seems
that the name for Plan 9 from User Space's implementation was suggested
by groff maintainer G. Brandon Robinson.

Not sure if the intention was to make these definitions compatible, but
it would be nice if they were.

Currently, Plan 9 from User Space's .MR expects its second argument to
be parenthesized.  groff's .MR does not.  This results in extra
parentheses appearing in manual references when viewing Plan 9 from User
Space's manual pages on a system using groff.

An experiment. Let's see if it's any good.
Also document the Mac conventions in devdraw(3).

Suggested by G. Brandon Robinson.

The overloading of IR emits magic \X'...' sequences that turn into HTML manual links.
But not all such IR invocations should be manual links;
those had to be written to avoid the IR macro before.
Worse, the \X'...' ending the IR causes troff to emit only a single space after a period.

Defining a new IM macro for manual references fixes both problems.

Fixes #441.

Alt X 1234 for U+1234
Alt X X 12345 for U+12345
Alt X X X 103456 for U+103456.

Rename following .cvsignore files to .gitkeep since they are
required by the build (directories must exist before build):
	- bin/fossil/.gitkeep
	- bin/fs/.gitkeep
	- bin/venti/.gitkeep

Change-Id: I9c2865058480cffb3a4613f25e2eca1f7e5578c0

Change-Id: I9130e7d10ae42e894923097a7c97da802eb0d8c3
Reviewed-on: https://plan9port-review.googlesource.com/2962
Reviewed-by: Russ Cox <rsc@swtch.com>

Change-Id: Id441d4df192c47388af6b5da306f14d90f066d18
Reviewed-on: https://plan9port-review.googlesource.com/1173
Reviewed-by: Russ Cox <rsc@swtch.com>