maint: accommodate gcc's -Wstrict-overflow option

maint: accommodate gcc's -Wstrict-overflow option

* src/factor.c (factor_using_pollard_rho): Change type of "i"
to unsigned to avoid warning from gcc's -Wstrict-overflow.
* src/expr.c: Use an unsigned intermediate.
* src/dircolors.c (main): Reorder operations to avoid the risk of
pointer overflow.
* src/tr.c (squeeze_filter): Change NOT_A_CHAR from an anonymous
"enum" to an "int", to avoid this warning:
tr.c:1624:10: error: assuming signed overflow does not occur when
  simplifying conditional to constant [-Werror=strict-overflow]
* src/pr.c (main): Make index "i" unsigned.