Imapflags: added validation and code support for variables.

src/lib-sieve/plugins/imapflags/Makefile.am

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/cmd-set.c

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

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

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

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

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

src/lib-sieve/sieve-code.c

src/lib-sieve/sieve-code.h