ld: pe: Apply review suggestions on the existing exports/imports arrays

ld: pe: Apply review suggestions on the existing exports/imports arrays

Use a separate explicit max_exports/imports field, instead of
deducing it from the number of allocated elements. Use a named
constant for the incremental growth of the array.

Use bool instead of int for boolean values.

Remove an unnecessary if statement/scope in the def_file_free
function.

Add more verbose comments about parameters, and about insertion
into an array of structs.

Generally use unsigned integers for all array indices and sizes.
The num_exports/imports fields are kept as is as signed integers,
since changing them to unsigned would require a disproportionate
amount of changes ti pe-dll.c to avoid comparisons between signed
and unsigned.

Simply use xrealloc instead of a check and xmalloc/xrealloc;
xrealloc can take NULL as the first parameter (and does a similar
check internally). (This wasn't requested in review though,
but noticed while working on the code.)