global: Use i_zero() instead of memset() where possible

src/lib-sieve-tool/sieve-tool.c

src/lib-sieve/plugins/special-use/tst-specialuse-exists.c

src/managesieve/managesieve-capabilities.c

src/managesieve/managesieve-client.c

src/plugins/doveadm-sieve/doveadm-sieve-cmd.c

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