lib-sieve: sieve-extensions - Use int for sieve_extension_register() status result

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

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

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

src/lib-sieve/sieve-extensions.c

src/lib-sieve/sieve-extensions.h

src/plugins/imapsieve/sieve-imapsieve-plugin.c

src/plugins/sieve-extprograms/sieve-extprograms-plugin.c

src/testsuite/testsuite-common.c