Rationalizing sed -i/-I (in-place editing) argument handling

A relatively minor but longstanding incompatibility between FreeBSD
and many other systems is the way sed handles backup files for
in-place editing -- sed's -I and -i options. GNU sed and other BSDs
accept an optional argument: -I.bak will save a backup file with a
.bak extension, and -I with no argument will not create a backup file.
FreeBSD currently accepts either -I.bak or -I .bak to save a backup
with the given extension, and -I "" (an empty argument) to specify no
backup.

I've been tripped up by this in the past and I know many others have.
Most recently tobik_at_  filed PR 254091 for this. Now, I think a single
change to make -i/-I to be compatible with other sed implementations
in one step is too much of a POLA violation, but I think it can
reasonably be done in stages:

1. Update the man page to indicate that -i/-I should not have a space
between the flag and the extension. This is compatible with current
FreeBSD sed, other BSDs sed, GNU sed, and my proposed changes below.
No backup is still a special case and remains as -I "".

I've opened https://reviews.freebsd.org/D29128 with proposed man page changes.

2. Continue accepting -I .bak, but emit a warning suggesting the use
of -I.bak instead.

3. Change -I/-i to getopt optional arguments, but keep a special case
for -I/-i "". At this point -I .bak becomes invalid as with other
seds, and specifying no backup can be done with either -I "" or -I
with no argument. This relies on an empty argument having no other
sensible interpretation.

4. Continue accepting -I "" to specify no backup, but emit a warning
suggesting the use of -I with no argument.

5. Retire the special case for -I "".

These steps could be done over an extended period of time (such as one
major release to another) - this is most important between steps 2 and
3.

Please let me know what you think, and if there's something I've missed.