Include extension: implemented global variables namespace.

TODO

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

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

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

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

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