add 'show environment', 'setenv', and 'unsetenv' commands

[stspdotname]

Being able to manage environment variables from within nsh will be useful, especially if nsh is used as a login shell because nsh's environment is visible to other commands run by nsh.

Setting variables can now affect the behaviour of nsh itself. For example, it is possible to set NSH_MANUAL_PAGE from within a running nsh session and the new value will immediately be used by the 'manual' command. This particular variable is only useful to nsh developers themselves but the same mechanism could be used for user-facing variables in the future.

If 'show environment' is run without further arguments all variables will be displayed. Otherwise the value of the specified variable will be displayed if the variable exists.

All commands support tab-completion for names which already exist in the environment. The 'setenv' commands appends "=" if the name being completed does not yet exist in the environment.

[stspdotname]

By the way, an 'appendenv' command couldl be added in a later commit if this one is accepted.

[smytht]

OKed by Chris, OK Tom snyth