[CLEANUP]: Added TRY_SYS_VAR_IMPORT() macro to env.h.
[CLEANUP]: Added comment to initialization of sh_status for clarity. Changed the name of report_error() to debug_error_info() globally.
[FEATURE]: Added imported and user_set flags to env_var_t. Amended add_env_var() to accept these new flags. Amended set_env_var() to set these flags accordingly. Amended show_env() to show these flags. Amended all add_env_var() calls to pass these flags.
[FEATURE]: Added typedef unsigned char shell_flags_t 'bitfield' to keep track of internal shell state. Added corresponding flag getting/setting functions set_shell_flag_{on,off}, get_shell_flag() to take care of this. Amended some functions to behave differently based on the setting of these flags.
[BUGFIX]: Added a check to add_env_var() to check if a variable doesn't already exist before adding it. Added ERR_VAR_EXISTS error code and string.
[FEATURE]: Added SHELL_FLAG_EXPORT to shell flags. Added check to export-var builtin command. Amended comment for SHELL_FLAG_RUNNING
[CLEANUP]: Renamed built-in commands to use 'var' instead of 'env'
[CLEANUP]: Moved debug_erro_info() and sizeof_array() macros from shell.h to helpers.h
[CLEANUP]: Moved error code macros for shell_error() from shell.h to helpers.h
[CLEANUP]: Amended sh_status initialization
[CLEANUP]: Rename show-var to show-vars