Resurrect standalone command

While command(1) is now classified by POSIX as an intrinsic utility and
no longer required to exist as a standalone program, it turns out that
we still have a use for it (cf. env(1)), so resurrect it.

PR: 291879
Fixes: 4100bd6caa66 ("usr.bin: Remove intrinsic utilities")
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D54341