rename find_envvar to getenv

This function is considered to have less relationship to strings, so it
lives in shell.c rather than string-util.c
add putenv into shell.c

Handler of "export" is reduced using putenv.

Issue: According to manual page, it returns ENOMEM on failure, but I do
       not find which header to include.
output hint using printf in shell_task
let USER be set on start

USER marks your identification, and can be checked by file system for
access control.
prevent writing USER through putenv

USER can only be changed by putenv_internal. Kernel has the rights to
use it directly, but user does not.
check access rights before returning file content

Issue: How to change mode of .account_config to 600? Its default mode in
       this commit seens to be 644.
fix bug of printing non-existing file name