Properly implemented circular include detection for the include extension.

src/lib-sieve/plugins/include/cmd-include.c

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

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

src/lib-sieve/plugins/include/included3.sieve

src/lib-sieve/sieve-lexer.c

src/lib-sieve/sieve-script.c

src/lib-sieve/sieve-script.h

src/lib-sieve/sieve.c