maint: simplify comparisons

maint: simplify comparisons

* src/comm.c (compare_files):
* src/join.c (keycmp):
* src/ls.c (off_cmp):
* src/ptx.c (compare_words, compare_occurs):
* src/set-fields.c (compare_ranges):
Prefer ((a > b) - (a < b)) to variants like (a < b ? -1 : a > b)
as it’s typically faster these days.