global: Replace realpath with path-util from core.

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

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

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

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

src/lib-sieve/util/Makefile.am

src/lib-sieve/util/realpath.c

src/lib-sieve/util/realpath.h

src/managesieve/main.c

src/testsuite/testsuite-mailstore.c

src/testsuite/testsuite.c