i18n: mark help strings for translation
The gettext function is just another name for the normal _ function
and it is used for translating docstrings when using _ would make
pygettext.py output a warning.
% basic
% unknown
*** [alias] unknown: command bargle is unknown
% ambiguous
*** [alias] ambiguous: command s is ambiguous
% recursive
*** [alias] recursive: circular dependency on recursive
adding foo
% with opts
C foo