The brace should be on a separate line.

buf needs to be freed after we're done with it, but only if it's not equal to d->d_buf.

-z needs to be documented in this usage string.

we can use bool and return true / false

FreeBSD style(9) wants doesn't use this style; should be

/*
 * Decompress a data section if needed (using ZLIB).
 * Returns 0 if sucessful, 1 otherwise.
 */
static int decompress_section(struct section *s, 
    unsigned char *compressed_data_buffer, uint64_t compressed_size, 
    unsigned char **ret_buf, uint64_t *ret_sz)
{

these should be indented by 1/2 tabs

{} are not required for single-statement bodies. They are permitted but need to be consistent throughout and they're not used elsewhere

Shouldn't it be Fl z | Fl -decompress?

"specified by .Fl x or .Fl p"

new_buf is uninitialized if (re->options & RE_Z) == 0.

The indentation is wonky.

Please avoid indenting goto labels.

free(NULL) is fine, there's no need to check for NULL.

Ditto.