maint: refactor common expand(1) and unexpand(1) code

maint: refactor common expand(1) and unexpand(1) code

* src/expand.c, src/unexpand.c: Move global variables from here...
* src/expand-common.h, src/expand-common.c: ... to here.
* src/expand.c, src/unexpand.c: (parse_tab_stops, validate_tab_stops,
next_file): Move identical functions to new module.
(add_tab_stop): Move to new module, including additional code from
'unexpand' (keeping max_column_width) which will have no effect in when
used in 'expand'. Refactor common next-column calculation code
into a new function 'get_next_tab_column'.
* src/local.mk: (src_expand_SOURCES, src_unexpand_SOURCES): Add
'expand-common.c'; (noinst_HEADERS): Add 'expand-common.h'.
* po/POTFILES.in: Add 'expand-common.c'.
* tests/misc/expand.pl: Add more tests.
* tests/misc/unexpand.pl: Likewise.
* TODO: Move conclusions to above test after investigation.