Improve file(1)'s ELF support.

Improve file(1)'s ELF support.

Read any PT_NOTE sections to look for NT_GNU_BUILD_ID or Android API
level notes. I deliberately didn't NT_GNU_ABI_TAG because it's noisy --
every Linux executable has one -- but not something most command-line
users will have any use for. (And you can ask readelf(1) anyway.)

Also read the section headers to implement "stripped"/"not stripped".

This patch removes "uses %d libs" because it was actually just counting
dynamic sections in the ELF file, and there are only 0 or 1 of those in
a valid ELF flie. (If you really want this functionality, you have to
*parse* the dynamic section looking for the DT_NEEDED entries. But that's
more of a job for readelf(1) than file(1).)