Revised implementation of the require command.

src/lib-sieve/cmd-require.c

src/lib-sieve/sieve-ast.c

src/lib-sieve/sieve-ast.h

src/lib-sieve/sieve-generator.c

src/lib-sieve/sieve-generator.h

src/lib-sieve/sieve-interpreter.c

src/lib-sieve/sieve-validator.c