optional section flags not handled right

[Jeff Johnson]

When fixing bug 1031, I found that elfchk only recognized one optional ELF
section flag: SHF_ALLOC. This was special-cased in the checker code. So, when
we needed to add some more optional flags, there was no mechanism to do so.

In the past, SHF_ALLOC was set as a required flag in the database where
appropriate, and the checker would set the flag in the ELF file's attributes if
needed when checking them. This is probably wrong, but OK in the LSB 3.2 time
frame, since the optional flags for 1031 are all related to each other, and in
practice probably won't be seen separately.

To fix this for real, we should do two things:

 - Store optional flags in the database separately from required flags.

 - When checking, mask off the optional flags, rather than set them, so they
don't have to be set in the required_flags field.