Make Sieve extension context code uniform and consistent

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

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

src/lib-sieve/plugins/duplicate/tst-duplicate.c

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

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

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

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

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

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

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

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

src/lib-sieve/plugins/mime/cmd-extracttext.c

src/lib-sieve/plugins/mime/ext-extracttext.c

...ib-sieve/plugins/spamvirustest/ext-spamvirustest-common.c

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

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

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

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

src/lib-sieve/plugins/vacation/ext-vacation-seconds.c

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