Imapflags: added execution support for variables.

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/imapflags-variables.sieve

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

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

src/lib-sieve/plugins/variables/sieve-ext-variables.h