Reworked how shelved files are verified.
If shelved files are to be verified, shelved changelists are processed
rather than verifying by association with depots. This now complies
with documented restriction per 'p4 help verify' that only the @=
revision specifier is valid when the '-S' flag is used.
If '-recent' is specifed, only the most recent N shelved changelists
are verified.