fix comment typos

Hi Paolo,

Using http://github.com/lyda/misspell-check
I ran this command:

git ls-files|misspellings -f -|perl -nl \
  -e '/^(.*?)\[(\d+)\]: (\w+) -> "(.*?)"$/ or next;' \
  -e '($file,$n,$l,$r)=($1,$2,$3,$4); $q="'\''"; $r=~s/$q/$q\\$q$q/g;'\
  -e 'print "sed -i $q${n}s!$l!$r!$q $file"'

Here's the raw output:

sed -i '2015s!benifit!benefit!' ChangeLog
sed -i '2139s!occurences!occurrences!' ChangeLog
sed -i '2168s!gratuitious!gratuitous!' ChangeLog
sed -i '2195s!siezed!sized","seized!' ChangeLog
sed -i '2841s!unwieldly!unwieldy!' ChangeLog
sed -i '3281s!equivelent!equivalent!' ChangeLog
sed -i '221s!seperated!separated!' build-aux/help2man
sed -i '187s!imcomplete!incomplete!' sed/execute.c
sed -i '96s!repetion!repetition!' testsuite/BOOST.tests
sed -i '88s!successfull!successful!' testsuite/runtests.c
sed -i '190s!preceeding!preceding!' testsuite/tst-pcre.c

I know help2man is external, so you can probably fix that one
by getting a newer version.  The rest look legit.
(Of course, re "siezed", you'll want to manually select "seized")