Made homedir aquisition a callback.

TODO

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

src/lib-sieve-tool/sieve-tool.h

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

src/lib-sieve/sieve-settings.h

src/lib-sieve/sieve-types.h

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

src/testsuite/testsuite.c