ueventd: don't double fork firmware handlers

ueventd may be asked to handle firmware during the time critical
coldboot process.  If we double fork to avoid needing to reap the
firmware handler, then we may add significant delay to this process,
as the first child may not get scheduled quickly enough for waitpid()
to complete without delay.

Bug: 63081260
Test: boot bullhead and sailfish, check that firmwares are loaded,
      no zombie ueventd processes remain, and no new errors are shown
Change-Id: I2bac3b1fbc3a58557a00326e491c104656db27ae