[Toybox] printenv -0

[enh]

https://github.com/landley/toybox/commit/52bbc1e0a410b44a926b04aaae3b00f9f50da81e
seems to have broken `printenv -0`. (`printenv --null` still works.)

/tmp/toybox$ ./toybox printenv --null LOGNAME | xxd
00000000: 656e 6800                                enh.
/tmp/toybox$ ./toybox printenv -0 LOGNAME | xxd
printenv: Unknown option '0' (see "printenv --help")

since printenv doesn't have other options, the workaround used in env
doesn't seem to apply...