encode_long, encode_timeval: mechanically replace exp with exponent

This helps fix a -Wshadow issue with exp(3) with tests/sys/acct/acct_test,
which include math.h, which in turn defines exp(3)

MFC after: 2 weeks
Tested with: clang, gcc 4.2.1, gcc 4.9
Sponsored by: Dell EMC Isilon