lib-sieve: Cleaned up command definitions using newer C99 syntax.

src/lib-sieve/cmd-discard.c

src/lib-sieve/cmd-if.c

src/lib-sieve/cmd-keep.c

src/lib-sieve/cmd-redirect.c

src/lib-sieve/cmd-require.c

src/lib-sieve/cmd-stop.c

src/lib-sieve/ext-envelope.c

src/lib-sieve/ext-fileinto.c

src/lib-sieve/ext-reject.c

src/lib-sieve/plugins/body/tst-body.c

src/lib-sieve/plugins/date/tst-date.c

src/lib-sieve/plugins/duplicate/tst-duplicate.c

src/lib-sieve/plugins/editheader/cmd-addheader.c

src/lib-sieve/plugins/editheader/cmd-deleteheader.c

src/lib-sieve/plugins/enotify/cmd-notify.c

src/lib-sieve/plugins/enotify/tst-notify-method-capability.c

src/lib-sieve/plugins/enotify/tst-valid-notify-method.c

src/lib-sieve/plugins/environment/tst-environment.c

src/lib-sieve/plugins/ihave/cmd-error.c

src/lib-sieve/plugins/ihave/tst-ihave.c