Imap4flags: properly named extension objects.

src/lib-sieve/plugins/imap4flags/cmd-flag.c

src/lib-sieve/plugins/imap4flags/ext-imap4flags-common.c

src/lib-sieve/plugins/imap4flags/ext-imap4flags-common.h

src/lib-sieve/plugins/imap4flags/ext-imap4flags.c

src/lib-sieve/plugins/imap4flags/tag-flags.c

src/lib-sieve/plugins/imap4flags/tst-hasflag.c

src/lib-sieve/sieve-extensions.c