Don't use non-standard ACCESSPERMS macro

[alyssais]

This macro is non-standard, and is not defined by e.g. musl libc. It's just a define for 0777, so just use that instead.

[paulh-aja]

After reviewing this PR with other software engineers on the internal AJA NTV2 team, a suggestion was made to do something like this:

#ifndef ACCESSPERMS
#define ACCESSPERMS 0777
#endif

And then leave the rest of the code as it was prior to the change in this PR.

[alyssais]

Where would make the most sense to define that? In some header, or in this file directly?