MFC r312119,r312216,r312226:

r312119:

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)

Tested with: clang, gcc 4.2.1, gcc 4.9

r312216:

Revert r312119 and reword the intent to fix -Wshadow issues
between exp(3) and exp var.

The approach taken previously was not ideal for multiple
functional and stylistic reasons.

Add to existing sed call in Makefile to replace exp with
exponent instead.

Requested by: bde

r312226:

Fix typo in r312216

I meant to replace "exp" with "exponent", not "expected"

Pointyhat to: ngie