Previous commit bf4e5c3e broke 2 things by returning too early when the
user defines a custom command:

- the "run as login shell" option becomes useless because we do not
  prepend argv0 with a "-" anymore
- argv0 itself should be the basename of the program run, but we don't
  do that anymore, and g_shell_parse_argv() does not do it either

This patch restores the old behavior by allowing the function to execute
to the end, even when defining a custom command.