global: Replace strncasecmp() with str_begins_icase*()

src/lib-sieve/cmp-i-ascii-casemap.c

src/lib-sieve/plugins/mime/tag-mime.c

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

src/lib-sieve/sieve-ast.h

src/lib-sieve/sieve-lexer.c

src/lib-sieve/sieve-storage.c

src/lib-sieve/storage/dict/sieve-dict-storage.c

src/lib-sieve/storage/file/sieve-file-storage.c

src/lib-sieve/storage/ldap/sieve-ldap-storage.c

src/managesieve-login/client.c

src/managesieve-login/managesieve-proxy.c

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