Cleanup _zed_event_add_nvpair()

When _zed_event_add_var() was updated to be the common routine
for adding zedlet environment variables, an additional snprintf()
was added to the processing of each nvpair.  This commit changes
_zed_event_add_nvpair() to directly call _zed_event_add_var()
for nvpair non-array types, thereby removing a superfluous call to
snprintf().  For consistency, the helper functions for converting
nvpair array types are similarly adjusted to add variables.

The _zed_event_value_is_hex() and _zed_event_add_var() functions have
been moved up in the file since forward declarations are not used,
but no changes have been made to these functions.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3042