doveadm: Code cleanup - add and use DOVEADM_PRINT_TYPE_PAGER macro

doveadm-print: Add "formatted" print formatter This allows using the header names as %{variables} in the format string.

doveadm: Add JSON formatter support

doveadm-print: Add flag indicating number-only field

doveadm: Changed most print formatters to write to ostream. This allows all the formatters to be used when sending replies to the upcoming doveadm HTTP server responses. The "table" formatter wasn't modified, because it writes to both stdout and stderr.

doveadm: Added -h parameter to hide header line from tab and table formatter output.

doveadm: Added doveadm_print_istream() for easily printing istreams.

doveadm: Added flag to specify which column table formatter expands. The default also was changed to last column, not first.

doveadm: Change "stats dump" default formatter to "tab".

doveadm: Added client/server architecture support for running mail commands. This is done when doveadm_worker_count is non-zero.

doveadm: Moved print formatters list to doveadm.c

doveadm: Added "pager" formatter and a function to output streamed output values. --HG-- branch : HEAD

doveadm: Output flushing fixes. --HG-- branch : HEAD

doveadm: Don't crash when mail command that doesn't print anything uses -A. --HG-- branch : HEAD

doveadm: Output is now written via "formatter" interface. The default can be changed with -f parameter. Currently implemented 3 formatters: flow, tab and table. --HG-- branch : HEAD