Several `const`s should be `bitflags`

[utkarshkukreti]

Some random examples (there are many more):

• MmapFlag
• MmapProt
• SockMessageFlags

[carllerche]

Looks like you are correct :)

[fiveop]

After #243 only flags in src/sys/socket/consts.rs should remain to be converted.

[kamalmarhubi]

@fiveop are you working on the remaining flags?

[fiveop]

I think we can close this now.

[kamalmarhubi]

@fiveop did you convert the ones in the original issue, or go over all flags? If the former, I think we could keep this open pending an audit to make sure we're using bitflags all over.

[fiveop]

I am pretty sure that I found all of them, but please do check again.

[kamalmarhubi]

@fiveop skimmed all the source, looks good!