Improvements for the brand detection and prioritization.

MFC r315701 (by ed):
Set the interpreter path to /nonexistent.

MFC r315749:
Adjust r314851 to not require every brand to specify interpreter path.

MFC r315753:
Add a flag BI_BRAND_ONLY_STATIC to specify that the brand only
matches static binaries.

MFC r315754:
Update r315753 with the proper flag name.

MFC r316211:
A followup to r315749, two more places where brand->interp_path was
accessed unconditionally.