nfscommon: Add support for an NFSv4 operation bitmap

NFSv4.1/4.2 uses operation bitmaps for various operations,
such as the SP4_MACH_CRED case for ExchangeID.
This patch adds support for operation bitmaps so that
support for SP4_MACH_CRED can be added to the NFSv4.1/4.2
server in a future commit.

This commit should not change any NFSv4.1/4.2 semantics.

MFC after: 3 months