rework help messages for fee-related options
rework help messages for fee-related options

- mention the units and show the default for -paytxfee

(cherry picked from commit 4aaa01783d4e3592db456fb2db05207c3c278244)

# Conflicts:
#	src/init.cpp

reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin