utils/nfsd: add support for minorvers4

minorvers4 can be used to either enable or disable nfsv4.x.

If minorvers4 is a positive integer n, in the allowed range (only
minorversion 1 is supported for now), the string "+4.n" is appended
to the versions string written onto /proc/fs/nfsd/versions.

Correspondingly, if minorver4 is a negative integer -n, the string
"-4.n" is written.

With the default value, minorvers4==0, the minor version
setting is not changed.

Note that unlike the protocol versions 2, 3, or 4.  The minor version
setting controls the *maximum* minor version nfsd supports.  Particular
minor version cannot be controlled on their own.  With only minor
version 1 supported at the moment the difference doesn't matter,
but for future minor versions greater than 1, enabling minor
version X will enable support for all minor versions 1 through X.
Disabling minor version X will disable support for minor
versions X and up, enabling 1 through X-1.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Steve Dickson <steved@redhat.com>