printenv: remove unsupported short options from getopt_long

printenv: remove unsupported short options from getopt_long

This patch only improves some error messages. Here is the behavior
before the patch:

    $ printenv -i
    Try 'printenv --help' for more information.
    $ printenv -u
    printenv: option requires an argument -- 'u'
    Try 'printenv --help' for more information.
    $ printenv -u a
    Try 'printenv --help' for more information.

Here is the behavior after:

    $ ./src/printenv -i
    ./src/printenv: invalid option -- 'i'
    Try './src/printenv --help' for more information.
    $ ./src/printenv -u
    ./src/printenv: invalid option -- 'u'
    Try './src/printenv --help' for more information.
    $ ./src/printenv -u a
    ./src/printenv: invalid option -- 'u'
    Try './src/printenv --help' for more information.

* src/printenv.c (main): Remove 'i' and 'u' from the short options given
to getopt_long.