Gopt broke wal_dir and snap_dir: they are no longer relative to work_dir

[Kostja Osipov]

After introduction of gopt, Tarantool main() chdir's to the working directory only after
option parsing, inlcuding --init-storage option. As a result, work_dir no longer applies
to --init-storage.

For example, provided the following directory layout:

/bug
/bug/snaps
/bug/xlogs

The following .cfg is not working correctly with --init-storage:
--quote: tarantool.cfg
slab_alloc_arena = 0.1

pid_file = "box.pid"

work_dir = "bug"
snap_dir = "snaps"
wal_dir = "xlogs"

logger="tee -a tarantool.log"

primary_port = 33013
secondary_port = 33014
admin_port = 33015

rows_per_wal = 50

namespace[0].enabled = 1
namespace[0].index[0].type = "HASH"
namespace[0].index[0].unique = 1
namespace[0].index[0].key_field[0].fieldno = 0
namespace[0].index[0].key_field[0].type = "NUM"
--end quote

[Kostja Osipov (kostja)]

Imported from JIRA-MY-1194.