Prefer other types to int in incremen.c

Prefer other types to int in incremen.c

* src/incremen.c (struct dumpdir_iter, dumpdir_first)
(read_incr_db_01, dumpdir_ok, list_dumpdir):
Prefer bool to int for booleans.  All uses changed.
(read_incr_db_01): Don’t assume getline returns <= INT_MAX.
(dumpdir_ok): Prefer char to int for chars.