ftabaddstring adds a string to the pclntab.
The pclntab uses C strings, so the code added 1 to the length.
However, it also added an extraneous 1 in the Grow call. Remove that.

While we're here, simplify, document, remove an unnecessary parameter,
and remove some unnecessary conversions.

Shaves off a few bytes here and there, and thus updates #6853.

file      before    after     Δ       %       
go        14671316  14659028  -12288  -0.084% 
addr2line 4280552   4276456   -4096   -0.096% 
api       6058936   6050744   -8192   -0.135% 
buildid   2861040   2856944   -4096   -0.143% 
cgo       4867912   4863816   -4096   -0.084% 
compile   25770104  25753720  -16384  -0.064% 
cover     5286888   5282792   -4096   -0.077% 
dist      3634048   3629952   -4096   -0.113% 
doc       4691000   4686904   -4096   -0.087% 
fix       3393736   3389640   -4096   -0.121% 
link      6109280   6105184   -4096   -0.067% 
nm        4225960   4221864   -4096   -0.097% 
objdump   4636520   4632424   -4096   -0.088% 
pack      2285200   2281104   -4096   -0.179% 
pprof     14657508  14645220  -12288  -0.084% 
test2json 2818568   2814472   -4096   -0.145% 
trace     11618524  11610332  -8192   -0.071% 
vet       8403544   8395352   -8192   -0.097% 

Change-Id: I20b1f541de5d3ed326dd937aad6a43801862df51
Reviewed-on: https://go-review.googlesource.com/c/go/+/171820
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>