readonly: Prints all environment variables with '-p' (should be empty)

[dylanaraps]

Running readonly -p in mrsh displays all environment variables when it should display nothing (if no variables were set as readonly).

Other shells:

-> ash
-> readonly -p
->
-> dash
-> readonly -p
->
-> yash
-> readonly -p
->

mrsh:

# Truncated as the output is long.
-readonly -p | head -6
readonly CMAKE_GENERATOR=Ninja
readonly DISPLAY=:0
readonly ENV=/home/goldie/.ashrc
readonly FFF_OPENER=opn
readonly HISTFILESIZE=10000
readonly HOME=/home/goldie

The prompt is also displayed incorrectly, however this is related to #88 and >.