find-esp: change "unprivileged_mode" parameter to be tristate

find-esp: change "unprivileged_mode" parameter to be tristate

Previously, unprivileged mode for find_esp_and_warn() and
find_xbootldr_and_warn() could be enabled or disabled. With this change
it can also be set to negative in which case the functions will enable
it automatically if found to be executing without privileges.

This just moves te geteuid() check we often do for the param inside of
the functions.

At the same time internally in the functions we also pass around the
VerifyESPFlags field across the various functions instead of booleans.

Both changes are just refactoring. No changes in behaviour.