Cleaned up imapflags extension.

src/lib-sieve/plugins/imapflags/cmd-addflag.c

src/lib-sieve/plugins/imapflags/cmd-removeflag.c

src/lib-sieve/plugins/imapflags/cmd-setflag.c

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

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

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

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

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