MFC r312119,r312216,r312226:

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@313214 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f